Simplify the signature of par_for_each_in
Simplify the signature of par_for_each_in

Given `T: IntoIterator`/`IntoParallelIterator`, `T::Item` is
unambiguous, so we don't need the explicit trait casting.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust