Rollup merge of #48115 - Centril:feature/iterator_flatten, r=alexcrichton

Add Iterator::flatten

This adds the trait method `.flatten()` on `Iterator` which flattens one level of nesting from an iterator or (into)iterators. The method `.flat_fmap(f)` is then redefined as `.map(f).flatten()`. The implementation of `Flatten` is essentially that of what it was for `FlatMap` but removing the call to `f` at various places.

Hopefully the type alias approach should be OK as was indicated / alluded to by @bluss and @eddyb in https://github.com/rust-lang/rfcs/pull/2306#issuecomment-361391370.

cc @scottmcm

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