replace `Random.nth` by `Iterators.nth` (#61486)
`Iterators.nth` has been added in #56580 for Julia 1.13. Currently
`Random` has its own (non-public) `nth` function, which is used for
`AbstractDict` and `AbstractSet`. This PR replaces it by
`Iterators.nth`. Apart from removing redundant code, this also allows
user-defined methods for `Iterators.nth` (which is public) to be used in
`rand`.
Note that I call `Iterators.nth` with `@inbounds`. This doesn't make any
difference for the default method (at least not for `AbstractDict` and
`AbstractSet`), but it may do so for user-defined methods. I'm having
`SmallBitSet` from my package SmallCollections.jl in mind, where `nth`
is just some bit juggling, see matthias314/SmallCollections.jl#17. When
filling a vector with random elements of a `SmallBitSet`, around 20% of
the runtime would be spent on checking bounds. Of course, I can remove
the `@inbounds` if you don't like it.
I've also dropped the `::eltype(iter)` return type indicator of
`Random.nth`. I've tested `Set{T}` where `T` is a `Union` with up to 6
elements, and also `Generator`. In all cases, `@code_typed` could
predict the return type.
