nat_split2 and pos_split2 are reversed

[just-max]

(documentation snippets are from https://c-cube.github.io/qcheck/0.18/qcheck-core/QCheck/Gen/index.html)
Expected behaviour: generators with "nat" in their names should generate non-negative natural numbers, generators with "pos" should generate strictly positive natural numbers.

The documentation agrees:

• val nat_split2 : int -> (int * int) t : ... generates pairs (n1, n2) of natural numbers ...
• val pos_split2 : int -> (int * int) t : ... generates pairs (n1, n2) of strictly positive (nonzero) natural numbers ...

What actually happens:

• nat_split2 cannot generate zeroes, even though it should (ie. QCheck.Gen.(generate (nat_split2 2)) only generates pairs (1, 1))
• pos_split2 can and does generate zeroes, even though it shouldn't (ie. QCheck.Gen.(generate (pos_split2 0)) does not cause errors and generates pairs (0, 0))
• The documentation for nat_split2 also states the restriction "raises Invalid_argument unless n >= 2" which should really apply to pos_split2.
• The documentation for pos_split2 refers to nat_split2 (... "nat_split2 n generates pairs")

Although it should probably be a separate issue, the counterparts generalized to lists, nat_split and pos_split, don't seem to work at all (regardless if the precondition n <= k is satisfied).

[gasche]

I am probably responsible for the mistakes (I proposed nat_split and pos_split in #114), I'll have a look.