Move Random to a splitting generator

[gasche]

(This is a tracking issue for Random+Multicore, which we could act upon now that Multicore is merged in trunk.)

There is consensus that Random should "split" its global generator state when a new domain is spawned. There are two aspects to this:

• the domain-local-state API needs to support "splitting" functions; this was prototyped in RFC: generalize the Domain.DLS interface to split PRNG state for child domains ocaml-multicore/ocaml-multicore#756, submitted upstream as generalize the Domain.DLS interface to split PRNG state for child domains #10887
• the PRNG should change to a new algorithm where a splitting operation has been studied to be statistically robust; see [RFC] changing the Random PRNG to a splittable algorithm from PRINGO RFCs#28 and Reimplementation of Random using an LXM pseudo-random number generator #10742

[xavierleroy]

Sir! Yes, sir! Right now I'm fixing a 2x slowdown in the bytecode interpreter (#10880) and trying to get the Inria CI working again, but after this I'll devote all my attention to the PRNG, sir!

[xavierleroy]

In the meantime, why don't you resurrect ocaml-multicore/ocaml-multicore#756 as a pull request here? We'll need DLS keys of the "split from parent" kind anyway.

[gasche]

Indeed, this is the first item above and I'm planning to do this soon. (Unfortunately I don't have a 2x speedup of my own to report ;-)

[gasche]

I have submitted the Domain.DLS change as an upstream PR: #10887.

[xavierleroy]

I think we're done with this task.