Introduce a config flag for upgrading to a world of custom PRNGs. The
flag defaults off, so that we can introduce custom PRNGs into the
codebase and allow downstream libraries time to upgrade.
Backwards compatible behavior is meant in an external sense. This does
not mean that our code is internally the same any longer.
A PRNG implementation is determined by a key shape and a set of basic
functions on such a key: seed, split, random_bits, and fold_in.
A PRNG implementation can then by lifted to an array-of-keys-like
object. Namely, a new internal pytree class PRNGKeyArray wraps the
implementation and maintains an array of keys of the right shape. This
array-like object is the new "key" that gets passed around the various
functions in the public random API (e.g. `random.uniform`,
`random.normal`, ...). So the PRNGKeyArray class really serves two
purposes at once:
1. To adapt key implementations into "arrays" of such keys.
2. To carry a reference to the PRNG implementation around and delegate
back to it from the functions in random.
