ditch fork!, add array version, do not support TaskLocalRNG
Now that seed! accepts an rng as 2nd arg, fork! is replaced by seed!.
Array version is useful for `@threads` (cf. docstring).
`fork(TaskLocalRNG())` is ambiguous: return a Xoshiro or TaskLocalRNG() ?
Leave that question for later.
Also, returning Xoshiro was seriously
complicating the docstring.
