Add missing tests of int{32,64}, option, and result combinators

[jmid]

Fixes #310

#307 and #308 added new combinators. This PR adds tests of them:

• unit tests for shrinkers
• positive property tests to exercise generators
• negative property tests (of false properties) to exercise generators, shrinkers, and printers
• stat distribution tests of generators

and even a couple of function tests of int{32,64},

[jmid]

All green and proof-read. Merging.