Further QCheck2 list shrinker improvements

[jmid]

This PR is a small adjustment to the QCheck2 list shrinker, so that lists counterexamples of size 1-3 are treated uniformly,
removing one element in turn. For example [1;2;3] will trigger [2;3], [1;3], [1;2] (in that order).

This makes a difference for the test suite where, e.g., Test bytes have unique chars failed will return minimal 2-char counterexamples as expected (rather than the 3-char ones that really bugged me).

With this, the QCheck2 shrinkers are now in good shape, witnessed by the expect test outputs on all OCaml{4,5} {64,32}-bit combinations.

The exception is the shrinker for QCheck2.Gen.list_size (and derivatives) which I've not attempted to improve.
For reference below I include a shrink_bench.log output for OCaml 4.14.2 on my laptop dominated by

• long_shrink - 2.070s
• lists shorter than 432 - 1.034s
• lists shorter than 4332 - 7.541s
• fold_left test, fun first - 1.908s

Of these the 3 first use QCheck2.Gen.list_size - and the last uses a function first (a known pain point also for QCheck(1)),

                                                         iteration seed 1234                   iteration seed 8743                   iteration seed 6789               total
Shrink test name                                  Q1/s  #succ/#att   Q2/s  #succ/#att   Q1/s  #succ/#att   Q2/s  #succ/#att   Q1/s  #succ/#att   Q2/s  #succ/#att    Q1/s   Q2/s
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
big bound issue59                                - skipped as generator is stateful, making it non-repeatable
long_shrink                                       0.027  149/352     0.782 3086/3148    0.006  147/345     0.563 3090/3150    0.008  145/339     0.725 3051/3111    0.040  2.070
ints arent 0 mod 3                                0.000   84/216     0.000   85/317     0.000   71/204     0.000   82/286     0.000   88/215     0.000    1/1       0.000  0.000
ints are 0                                        0.000   62/63      0.000   60/121     0.000   61/62      0.000   61/122     0.000   62/63      0.000   59/119     0.000  0.000
int32s arent 0l rem 3l                            0.000   41/101     0.000    1/1       0.000   38/109     0.000    1/1       0.000   35/116     0.000    1/1       0.000  0.000
int32s are 0l                                     0.000   31/32      0.000   30/61      0.000   31/32      0.000   30/61      0.000   28/29      0.000   27/55      0.000  0.000
int64s arent 0L rem 3L                            0.000   90/282     0.000    1/1       0.000   77/203     0.000    1/1       0.000   95/330     0.000    1/1       0.000  0.000
int64s are 0L                                     0.000   63/64      0.000   62/125     0.000   63/64      0.000   62/125     0.000   60/61      0.000   59/119     0.000  0.000
ints < 209609                                    - skipped as generator is stateful, making it non-repeatable
nat < 5001                                        0.000    6/56      0.000    7/77      0.000    6/47      0.000    7/69      0.000    3/24      0.000    8/85      0.000  0.000
char never produces 'abcdef'                      0.000    3/3       0.000    1/1       0.000    0/0       0.000    0/0       0.000    0/0       0.000    0/0       0.000  0.000
printable never produces '!"#$%&'                 0.000    2/15      0.000    1/10      0.000    1/10      0.000    2/15      0.000    2/14      0.000    1/10      0.000  0.000
printable never produces less than '5             0.000    3/3       0.000    1/1       0.000    2/2       0.000    1/1       0.000    3/3       0.000    1/1       0.000  0.000
bytes are empty                                   0.000   15/24      0.000    7/8       0.000   20/29      0.000   10/11      0.000    7/15      0.000    4/5       0.000  0.000
bytes never has a \000 char                       0.000    8/22      0.002   13/27      0.001   17/34      0.003   14/26      0.000    6/18      0.000    8/21      0.001  0.005
bytes never has a \255 char                       0.000   14/34      0.000    6/18      0.000   15/30      0.000    9/22      0.001   21/44      0.000    6/18      0.001  0.000
bytes have unique chars                           0.000   13/38      0.000   13/50      0.001   15/39      0.000    9/34      0.000    5/21      0.000   16/31      0.001  0.000
strings are empty                                 0.000   15/24      0.000    7/8       0.000   20/29      0.000   10/11      0.000    7/15      0.000    4/5       0.000  0.000
string never has a \000 char                      0.000    8/22      0.002   13/27      0.000   17/34      0.002   14/26      0.000    6/18      0.000    8/21      0.000  0.004
string never has a \255 char                      0.000   14/34      0.000    6/18      0.001   15/30      0.000    9/22      0.001   21/44      0.000    6/18      0.002  0.000
strings have unique chars                         0.000   13/38      0.000   13/50      0.002   15/39      0.000    9/34      0.000    5/21      0.000   16/31      0.002  0.000
pairs have different components                   0.000    0/4       0.000    0/6       0.000    0/4       0.000    0/6       0.000    0/6       0.000    0/6       0.000  0.000
pairs have same components                        0.000  125/126     0.000   63/124     0.000  124/125     0.000   64/126     0.000  119/120     0.000   63/125     0.000  0.000
pairs have a zero component                       0.000  124/188     0.000  123/307     0.000  123/186     0.000  124/310     0.000  118/182     0.000  123/308     0.000  0.000
pairs are (0,0)                                   0.000  125/126     0.000   63/124     0.000  124/125     0.000   64/126     0.000  119/120     0.000   63/125     0.000  0.000
pairs are ordered                                 0.000  827/17624   0.000   94/1192    0.000  690/12888   0.000   91/1059    0.000  687/13960   0.000   88/1033    0.001  0.000
pairs are ordered reversely                       0.000  124/125     0.000   62/124     0.000  123/124     0.000   61/122     0.000  122/123     0.000   62/124     0.000  0.000
pairs sum to less than 128                        0.000  116/129     0.000   58/130     0.000  120/146     0.000   58/137     0.000  119/141     0.000   55/126     0.000  0.000
pairs lists rev concat                            0.020  147/350     0.000   66/129     0.008  135/334     0.000   79/143     0.000  131/320     0.000   70/134     0.028  0.000
pairs lists no overlap                            0.001   26/48      0.000   13/15      0.000   18/39      0.000   17/20      0.001    8/18      0.000    9/28      0.001  0.000
triples have pair-wise different components       0.000    7/23      0.000    2/2       0.000    6/6       0.000    1/1       0.000    2/4       0.000    3/3       0.000  0.000
triples have same components                      0.000  188/252     0.000   65/129     0.000  177/240     0.000   66/130     0.000  182/246     0.000   62/123     0.000  0.000
triples are ordered                               0.000  188/252     0.000    4/5       0.000  177/178     0.000    5/6       0.000  187/250     0.000    3/4       0.000  0.000
triples are ordered reversely                     0.000  188/189     0.000  125/248     0.000  177/240     0.000   64/126     0.000  182/183     0.000   64/126     0.000  0.000
quadruples have pair-wise different components    0.000   23/35      0.000    3/3       0.000   11/11      0.000    4/19      0.000   14/38      0.000    2/2       0.000  0.000
quadruples have same components                   0.000  250/377     0.000  125/311     0.000  237/424     0.000  127/315     0.000  242/425     0.000  126/313     0.000  0.000
quadruples are ordered                            0.000  251/315     0.000    5/6       0.000  239/240     0.000    6/7       0.000  244/308     0.000    5/6       0.000  0.000
quadruples are ordered reversely                  0.000  251/252     0.000  126/249     0.000  239/302     0.000   63/123     0.000  244/245     0.000   66/128     0.000  0.000
forall (a, b) in nat: a < b                       0.000   13/22      0.000    1/1       0.000   10/15      0.000    5/14      0.000    5/6       0.000    2/5       0.000  0.000
forall (a, b, c) in nat: a < b < c                0.000   15/21      0.000    2/2       0.000   26/52      0.000    3/3       0.000    9/9       0.000    5/10      0.000  0.000
forall (a, b, c, d) in nat: a < b < c < d         0.000   23/28      0.000    3/3       0.000   30/55      0.000    4/4       0.000   13/13      0.000    4/4       0.000  0.000
forall (a, b, c, d, e) in nat: a < b < c < d < e  0.000   28/28      0.000    4/4       0.000   33/33      0.000    4/4       0.000   14/14      0.000    5/5       0.000  0.000
forall (a, b, c, d, e, f) in nat: a < b < c < d   0.000   30/30      0.000    5/5       0.000   38/38      0.000    6/6       0.000   16/16      0.000    6/6       0.000  0.000
forall (a, b, c, d, e, f, g) in nat: a < b < c <  0.000   31/31      0.000    6/6       0.000   41/41      0.000    7/7       0.000   22/22      0.000    7/7       0.000  0.000
forall (a, b, c, d, e, f, g, h) in nat: a < b <   0.000   35/35      0.000    7/7       0.000   48/48      0.000    8/8       0.000   22/22      0.000    7/7       0.000  0.000
forall (a, b, c, d, e, f, g, h, i) in nat: a < b  0.000   42/42      0.000    8/8       0.000   55/55      0.000    9/9       0.000   26/26      0.000    9/9       0.000  0.000
bind ordered pairs                                0.000  125/125     0.000    1/1       0.000  124/124     0.000    1/1       0.000  120/120     0.000    1/1       0.000  0.000
bind list_size constant                           0.000   49/200     0.000   12/25      0.000   50/197     0.000   12/25      0.000   51/205     0.000   13/28      0.000  0.000
lists are empty                                   0.000   12/18      0.000    7/8       0.003   17/23      0.000   10/11      0.000    4/9       0.000    3/4       0.003  0.000
lists shorter than 10                             0.000   39/213     0.000   18/34      0.000   53/277     0.000   16/22      0.000   47/283     0.000   14/28      0.001  0.000
lists shorter than 432                            0.145 1632/17856   0.509  403/429     0.162 1677/18346   0.205  406/441     0.053 1735/19013   0.321  412/455     0.360  1.034
lists shorter than 4332                           0.091   13/67      2.724 4022/4068    0.082   10/35      2.480 3987/4046    0.009    8/55      2.337 4001/4061    0.181  7.541
lists have unique elems                           0.000    8/17      0.000    8/23      0.002   12/20      0.000    8/19      0.000    8/17      0.000    4/17      0.003  0.000
int option are none                               0.000   11/23      0.000    1/3       0.000    9/19      0.000    1/3       0.000   11/23      0.000    1/3       0.000  0.000
int option are some 100 or more                   0.000    1/1       0.000    1/1       0.000    0/0       0.000    0/0       0.000    0/0       0.000    0/0       0.000  0.000
(int,string) result are Ok                        0.000    0/0       0.000    4/4       0.000    2/2       0.000    4/4       0.000    6/6       0.000    4/4       0.000  0.000
(int,string) result are Error                     0.000   11/11      0.000    1/1       0.000    9/9       0.000    1/1       0.000   11/11      0.000    1/1       0.000  0.000
tree contains only 42                             0.000   10/10      0.000    1/1       0.000   10/10      0.000    2/2       0.000   12/13      0.000    2/2       0.000  0.000
sum list = 0                                      0.000    0/0       0.000    0/0       0.000    0/0       0.000    0/0       0.000    0/0       0.000    0/0       0.000  0.000
fail_pred_map_commute_int                         0.000   77/238     0.000  107/364     0.000   74/228     0.000   19/69      0.000   87/269     0.000   28/71      0.000  0.001
fail_pred_map_commute_int32                       0.001  149/553     0.000   57/193     0.001  113/446     0.001  113/360     0.000  106/380     0.001   71/212     0.002  0.002
fail_pred_map_commute_int64                       0.001  181/600     0.001  107/383     0.001  184/575     0.001  136/404     0.001  181/536     0.002  133/408     0.002  0.004
fail_pred_strings                                 0.000    2/6       0.000    1/3       0.000    1/5       0.000    2/4       0.000    1/5       0.000    2/4       0.001  0.000
fold_left fold_right                              0.000   34/104     0.000   20/43      0.000   39/120     0.000   16/37      0.000   18/54      0.000   15/43      0.001  0.001
fold_left fold_right uncurried                    0.003   44/198     0.000   17/142     0.037   55/170     0.003   26/56      0.000    5/14      0.000   17/84      0.040  0.003
fold_left fold_right uncurried fun last           0.000   34/101     0.001   48/157     0.000   42/129     0.000   16/38      0.000   19/57      0.000   19/74      0.001  0.001
fold_left test, fun first                         0.002   36/65      0.322   87/25914   1.052   87/1062    1.482  395/20346   0.028   76/673     0.103   40/5860    1.081  1.908
                                                                                                                                                                    1.753 12.578