Add an exponential generator

[jmid]

This PR adds an exponential generator to both QCheck and QCheck2, using a mapping from a uniform [0;1] distribution
https://en.wikipedia.org/wiki/Relationships_among_probability_distributions
https://en.wikipedia.org/wiki/Exponential_distribution#Random_variate_generation

All of them takes a mean/average parameter, e.g., exponential 10., and results in a distribution where samples approximate that mean/average:

+++ Stats for exponential 10. dist ++++++++++++++++++++++++++++++++++++++++++++++++++++++++

stats dist:
  num: 5000, avg: 9.56, stddev: 9.91, median 7, min 0, max 89
    0..  4: #######################################################        1934
    5..  9: ##################################                             1202
   10.. 14: ####################                                            727
   15.. 19: ############                                                    452
   20.. 24: ########                                                        284
   25.. 29: ####                                                            164
   30.. 34: ##                                                              103
   35.. 39: #                                                                51
   40.. 44:                                                                  26
   45.. 49:                                                                  24
   50.. 54:                                                                  15
   55.. 59:                                                                   7
   60.. 64:                                                                   3
   65.. 69:                                                                   2
   70.. 74:                                                                   2
   75.. 79:                                                                   1
   80.. 84:                                                                   1
   85.. 89:                                                                   2
   90.. 94:                                                                   0
   95.. 99:                                                                   0

We've found the combinator useful in QCheck-STM to generate cmd lists of a reasonable length ocaml-multicore/multicoretests#472 but thought the combinator could be more generally useful.

We may even consider switching our list, array, string, and bytes generators to output lengths following such an exponential distribution, but that will be for another day - and PR...

[jmid]

Rebased on main after merging #299