Perform packing/unpacking of arguments and results in HDL

Running three experiments with the old and the new code shows that
avoiding packing/unpacking in Haskell yields a ~20-30% performance
increase for longer running tests. Table below shows the minimum
value for all runs.

| Test                       | main (min) | branch (min) | faster  | faster (>1s) |
|----------------------------|------------|--------------|---------|--------------|
| dyn_dispatch (debug)       | 0.34       | 0.31         | 8.82%   |              |
| ebreak_exception (debug)   | 1.42       | 1.08         | 23.94%  | 23.94%       |
| fpu_test (debug)           | 3.42       | 2.4          | 29.82%  | 29.82%       |
| hello_world (debug)        | 0.47       | 0.38         | 19.15%  |              |
| long_string (debug)        | 4.97       | 3.37         | 32.19%  | 32.19%       |
| loop_write (debug)         | 9.7        | 6.64         | 31.55%  | 31.55%       |
| many_calls (debug)         | 0.93       | 0.74         | 20.43%  | 20.43%       |
| print_a (debug)            | 0.31       | 0.31         | 0.00%   |              |
| print_b (debug)            | 0.39       | 0.32         | 17.95%  |              |
| print_numbers (debug)      | 3.8        | 2.71         | 28.68%  | 28.68%       |
| single_write (debug)       | 0.82       | 0.6          | 26.83%  |              |
| dyn_dispatch (release)     | 0.2        | 0.24         | -20.00% |              |
| ebreak_exception (release) | 0.27       | 0.2          | 25.93%  |              |
| fpu_test (release)         | 0.71       | 0.5          | 29.58%  |              |
| hello_world (release)      | 0.2        | 0.19         | 5.00%   |              |
| long_string (release)      | 0.35       | 0.25         | 28.57%  |              |
| loop_write (release)       | 0.95       | 0.8          | 15.79%  |              |
| many_calls (release)       | 0.27       | 0.28         | -3.70%  |              |
| print_a (release)          | 0.19       | 0.23         | -21.05% |              |
| print_b (release)          | 0.16       | 0.17         | -6.25%  |              |
| print_numbers (release)    | 0.49       | 0.43         | 12.24%  |              |
| single_write (release)     | 0.3        | 0.23         | 23.33%  |              |
| JTAG                       | 33.33      | 25.89        | 22.32%  | 22.32%       |
| JTAG chain                 | 126.67     | 89.85        | 29.07%  | 29.07%       |
|                            |            |              | 15.84%  | 27.25%       |

Figures are taken by observing clash-vexriscv-sim:unittests's times.

clash-vexriscv/clash-vexriscv.cabal

@@ -109,6 +109,7 @@ library
   default-language: Haskell2010
   exposed-modules:
     VexRiscv
+    VexRiscv.BlackBox
     VexRiscv.ClockTicks
     VexRiscv.FFI
     VexRiscv.JtagTcpBridge
@@ -119,18 +120,23 @@ library
   build-depends:
     base,
     bytestring >= 0.10 && < 0.13,
+    clash-lib,
     clash-prelude >= 1.9.0,
     clash-protocols,
     containers,
     directory >= 1.3 && < 1.4,
     filepath,
     Glob,
+    infinite-list,
+    mtl,
     network,
+    pretty-show,
     process >= 1.6 && < 1.8,
     random,
     string-interpolate,
     tagged,
     template-haskell,
+    text,
   extra-libraries: VexRiscvFFI, stdc++
   include-dirs: src/