Implement Dynarray.sort

[Jeannalyse]

This PR adds new standard library functions : Dynarray.stable_sort_sub, Dynarray.stable_sort and Dynarray.sort.
These functions were suggested here : #13343 (comment) .
They use Array.stable_sort_sub, so the sorting algorithm is the same as in Array. They consider a change in length to be a programming error, and raise an exception when they detect one.

Note that Dynarray.sort is, in fact, an alias for Dynarray.stable_sort : I figured that a default sorting function was a good thing to have, and that it could be later replaced by an other algorithm if needed.

I have modified the tests in testsuite/tests/misc/sort_sub.ml so that they test both Array.stable_sort_sub and Dynarray.stable_sort_sub.

I came up with three versions in total, each committed separately :

• version 1 : adapts the code in Array by using Dynarray.get, Dynarray.set, Dynarray.blit etc.
• version 2 : adapts the code in Array by using unsafe_get on the backing arrays
• version 3 : directly uses Array.stable_sort_sub on the backing array

My benchmarks show that the third one is the fastest, and it is also the shortest code-wise, so I only kept this one in the final commit. The benchmarks can be found on my fork on the branch named benchmarks/dynarray-stable-sort-sub. Here are some results :

Command	Mean [s]	Min [s]	Max [s]	Relative
./benchmarks.opt 1	1.509 ± 0.019	1.490	1.547	1.24 ± 0.03
./benchmarks.opt 2	1.359 ± 0.021	1.333	1.402	1.12 ± 0.03
./benchmarks.opt 3	1.213 ± 0.024	1.188	1.260	1.00

Please don't hesitate to ask questions and suggest improvements.

[nojb]

cc @ocaml/stdlib