Rollup merge of #23462 - alexcrichton:stabilize-cloned, r=aturon

 This commit stabilizes the `cloned` iterator after tweaking the signature to
require that the iterator is over `&T` instead of `U: Deref<T>`. This method has
had time to bake for awhile now and it's not clear whether the `Deref` bound is
worth it. Additionally, there aren't clear conventions on when to bound and/or
implement the `Deref` trait, so for now the conservative route is to require
references instead of `U: Deref<T>`.

To change this signature to using `Deref` would technically be a
backwards-incompatible change, but it is doubtful that any code will actually
break in practice.