Add a parallelizable PRNG

[edre]

It would be simple to write a seedable PRNG as a rayon Producer, at least with LCGs that support efficient skipping. Any user that does not care about determinism should probably stick with rand::thread_rng, which shouldn't have any concurrency bottlenecks.

A basic implementation would only emit u32s, but users may want other types or distributions, and incorporating those into the API would be more complicated.

[svenstaro]

Isn't this done as of #571?

[edre]

This issue is about a deterministic random Producer, which is different from the internal non-deterministic rng from #571.

[svenstaro]

You're right, I misread at first.