Random: allow seeding from an RNG (#51533)
This is the combination of two changes:
1) Allow seeding an RNG from another RNG, e.g. `seed!(Xoshiro(),
RandomDevice())`, because it's convenient, in particular for RNGs which
need a variable number of random bytes (think "rejection sampling").
2) In turn, it's convenient to only having to define this one method for
a given RNG type, and delegate handling of other kind of seeds (integer,
strings, ...) to another object: `SeedHasher(seed)` is a new RNG which
produces random streams from such seeds, with the help of SHA2; this
essentially gives an RNG API to what we are already using to transform
integer seeds for `Xoshiro` and `MersenneTwister` into vectors suitable
for their initialization.
While `SeedHasher` shouldn't be considered as a CSPRNG, it should have
very good statistical properties; if we go with this PR, I will first
run PractRand (looks good so far) and BigCrush tests before merging.
The API is not changed in this PR, `SeedHasher` remains internal; the goal
is to experiment more with it before commiting to an API, possibly in
v1.13.
