[feature] `FromParallelIterator` for `arrayvec::ArrayVec`

[BartMassey]

Would sure be nice to have rayon::iter::FromParallelIterator implemented for ArrayVec from the arrayvec crate. Would the implementation belong in Rayon under a feature gate, or somewhere else? What might make it hard? Should I just try to put together a pull request?

[cuviper]

Usually our trait implementations get added as an optional "rayon" feature of the other crate, especially because efficient implementations may need to access private details of the type.

[BartMassey]

Thanks much! Filed bluss/arrayvec#209