randomness-altering Threefry RNG upgrade

[froystig]

Tracking issue for upgrading through breaking changes to the Threefry PRNG implementation. Specifically, these changes alter the pseudorandom bits produced by a given key, relative to the current implementation. The upgrade at least includes #13040 (superseding #13012) and #13341, which are currently guarded by jax.config.jax_threefry_partitionable.

It may be convenient to offer backwards opt-in in the form of a PRNG implementation "snapshot" of Threefry, since all changes sit behind the PRNG interface (see #6899).

We can close this issue when the upgrade is complete, in that the new Threefry implementation is on by default.

[jakevdp]

I'd like the changes in #12539 to be part of this as well, under whatever flag is appropriate. What do you think?