The implementation in stdlib/random.ml is going to change.

…M family

As described in the paper [LXM: Better Splittable Pseudorandom Number
Generators (and Almost as Fast)](https://doi.org/10.1145/3485525) by
Guy L. Steele Jr. and Sebastiano Vigna, proceedings of OOPSLA 2021.

This provides a fast, state-of-the-art PRNG with full support for splitting.

Initialization from a seed (an array of integers) proceeds by
serializing the array to a byte sequence (each seed element occupies 8
bytes in little-endian representation) then hashing twice using the
MD5 hash function and two different suffixes, obtaining 256 bits of
initialization data.

More tests were added in testsuite/tests/lib-random, and the lone test
testsuite/tests/basic-more/testrandom.ml was moved there.

`@since` tags, etc.