Fix function generation reproducability

[jmid]

This PR fixes#236

QCheck and QCheck2 generates functions by inhabiting (and side-effecting) an underlying Hashtbl, packaged as Poly_tbl.
A new function application then triggers a generator call and adds the new binding to the underlying table.
Overall, since the generator call side-effects the state of the underlying Random.State, the number of function applications may then unknowingly affect the output of another generator, e.g., if the function generator is part of a tuple as the example in #236:

(triple small_int (fun1 Observable.int small_int) small_int)

This side-effecting clearly breaks (what we tend to think of as) independence of the individual tuple entry generators.

The PR fixes the function generator by splitting the Random.State and lets the later function applications side-effect the split copy instead, thus restoring (some form of) independence. We do so for both QCheck and QCheck2.

The RS.split is part of the Random.State interface of the forth-coming OCaml 5.0 release - but we add a "poor man's" split for backwards compatibility on <5.0.

Finally the PR adds tests - with repeated reruns. Without the fix both of them gave rise to different prints when repeated. Both are fixed with the proposed patch.

[jmid]

With a rewamped shrinker performance benchmark, below is the output of running it on this PR's fixed function generators.

When added up all the timing this PR represents

• a speed-up for the QCheck2 shrinkers - test fold_left fold_right uncurried run with seed 8743 is reduced from 9.131s to 2.225s accounting for most of the QCheck2 improvement
• a slow-down for the QCheck shrinkers - test fold_left test, fun first run with seed 8743 goes from 20.486s to 43.448s and from 125 successful shrink attempt out of 27750 to 191 out of 44563 attempts. That test run alone accounts for a 23sec slow-down (it is fixed by Shrinker improvements #235).

 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 0.023 149/351 0.917 3039/3099 0.005 148/349 0.594 3068/3127 0.008 146/345 0.673 3063/3124 0.036 2.184
ints arent 0 mod 3 0.000 84/216 0.000 2/2 0.000 71/204 0.000 1/1 0.000 88/215 0.000 88/305 0.000 0.000
ints are 0 0.000 62/63 0.000 61/123 0.000 61/62 0.000 61/122 0.000 62/63 0.000 61/123 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 0/0 0.000 1/1 0.000 0/0 0.000 0/0 0.000 0/0 0.000 0/0 0.000 0.000
strings are empty 0.000 249/250 0.000 8/16 0.030 4466/4467 0.001 13/26 0.000 0/1 0.000 1/2 0.030 0.001
string never has a \000 char 0.000 25/40 0.000 22/167 0.088 4466/4519 0.001 56/254 0.000 15/18 0.000 15/48 0.088 0.001
string never has a \255 char 0.001 249/316 0.001 59/318 0.090 4466/4520 0.002 97/529 0.378 9260/9365 0.002 41/194 0.469 0.005
strings have unique chars 0.003 248/269 0.000 18/30 0.922 4465/4536 0.002 24/52 0.000 14/34 0.000 15/20 0.925 0.002
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 0/10 0.000 0.000
pairs have same components 0.000 125/126 0.000 63/125 0.000 124/125 0.000 62/123 0.000 119/120 0.000 63/125 0.000 0.000
pairs have a zero component 0.000 124/188 0.000 122/306 0.000 123/186 0.000 122/306 0.000 118/182 0.000 123/308 0.000 0.000
pairs are (0,0) 0.000 125/126 0.000 63/125 0.000 124/125 0.000 62/123 0.000 119/120 0.000 63/125 0.000 0.000
pairs are ordered 0.000 827/17626 0.000 94/1217 0.000 690/12946 0.000 85/865 0.000 687/13963 0.000 94/1326 0.001 0.001
pairs are ordered reversely 0.000 124/125 0.000 62/124 0.000 123/124 0.000 62/124 0.000 122/123 0.000 62/124 0.000 0.000
pairs sum to less than 128 0.000 116/129 0.000 56/126 0.000 120/146 0.000 59/138 0.000 119/141 0.000 57/130 0.000 0.000
pairs lists rev concat 0.014 140/332 0.009 83/168 0.008 137/335 0.002 75/152 0.000 130/318 0.000 67/136 0.022 0.011
pairs lists no overlap 0.001 22/47 0.003 27/60 0.000 17/41 0.002 18/41 0.000 6/20 0.000 11/28 0.001 0.005
triples have pair-wise different components 0.000 7/31 0.000 3/15 0.000 6/6 0.000 3/3 0.000 2/6 0.000 3/3 0.000 0.000
triples have same components 0.000 188/252 0.000 64/127 0.000 177/240 0.000 64/128 0.000 182/246 0.000 62/122 0.000 0.000
triples are ordered 0.000 188/252 0.000 3/4 0.000 177/178 0.000 3/4 0.000 187/250 0.000 91/1021 0.000 0.000
triples are ordered reversely 0.000 188/189 0.000 64/126 0.000 177/240 0.000 124/247 0.000 182/183 0.000 65/127 0.000 0.000
quadruples have pair-wise different components 0.000 23/41 0.000 4/4 0.000 11/11 0.000 4/4 0.000 14/38 0.000 4/11 0.000 0.000
quadruples have same components 0.000 250/377 0.000 126/313 0.000 237/424 0.000 115/292 0.000 242/425 0.000 123/307 0.000 0.000
quadruples are ordered 0.000 251/315 0.000 5/6 0.000 239/240 0.000 4/5 0.000 244/308 0.000 5/6 0.000 0.000
quadruples are ordered reversely 0.000 251/252 0.000 66/128 0.000 239/302 0.000 126/250 0.000 244/245 0.000 66/128 0.000 0.000
forall (a, b) in nat: a < b 0.000 13/23 0.000 6/16 0.000 10/15 0.000 6/15 0.000 5/6 0.000 4/7 0.000 0.000
forall (a, b, c) in nat: a < b < c 0.000 15/22 0.000 3/7 0.000 26/53 0.000 7/28 0.000 9/9 0.000 3/3 0.000 0.000
forall (a, b, c, d) in nat: a < b < c < d 0.000 23/29 0.000 4/4 0.000 30/56 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 5/5 0.000 33/33 0.000 5/5 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 6/6 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 7/7 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 8/8 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 9/9 0.000 55/55 0.000 9/9 0.000 26/26 0.000 8/8 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 50/358 0.000 12/26 0.000 48/338 0.000 12/25 0.000 48/342 0.000 11/21 0.000 0.000
lists are empty 0.000 11/16 0.000 8/16 0.001 19/27 0.004 13/26 0.000 4/9 0.000 1/2 0.002 0.004
lists shorter than 10 0.000 50/1198 0.000 16/30 0.000 71/1637 0.002 21/42 0.000 36/868 0.000 15/29 0.001 0.002
lists shorter than 432 6.390 1696/5118102 1.069 412/457 6.171 1612/4863421 1.015 405/450 6.165 1667/5037661 0.207 419/447 18.727 2.291
lists shorter than 4332 2.240 13/190735 3.577 4022/4087 1.609 11/126052 4.343 4020/4067 1.467 7/126607 2.690 4013/4055 5.316 10.610
lists equal to duplication 0.152 20/23 0.494 4/7 0.000 7/13 0.000 3/6 0.021 20/25 0.105 17/35 0.173 0.600
lists have unique elems 0.000 7/17 0.000 11/22 0.002 12/44 0.004 17/30 0.000 6/16 0.000 10/20 0.003 0.004
tree contains only 42 0.000 10/10 0.000 2/2 0.000 10/10 0.000 2/2 0.000 12/13 0.000 2/2 0.000 0.000
fail_pred_map_commute 0.000 107/453 0.001 122/342 0.000 108/317 0.000 19/83 0.000 118/548 0.000 14/39 0.001 0.001
fail_pred_strings 0.000 1/3 0.000 2/5 0.000 1/2 0.000 1/4 0.000 1/2 0.000 1/4 0.000 0.000
fold_left fold_right 0.000 24/74 0.001 56/189 0.000 34/149 0.000 21/58 0.000 21/53 0.000 22/64 0.001 0.001
fold_left fold_right uncurried 2.541 97/80630 0.043 376/1550 0.145 38/390 2.225 2064/8057 0.000 5/20 0.000 4/17 2.686 2.269
fold_left fold_right uncurried fun last 0.001 21/88 0.001 56/189 0.000 35/220 0.002 72/170 0.000 22/73 0.000 28/73 0.001 0.003
fold_left test, fun first 0.001 40/57 0.001 15/28 43.448 191/44563 3.519 47/9773 11.188 223/75912 0.003 36/64 54.637 3.523
83.119 21.518