More cleanup from "impl Iter" based sources

More cleanup from "impl Iter" based sources

Not having to explicitly name the iterator return types saves
the need to name the returned iterator's reference lifetimes, if any.
This removes the need to implement Source on a reference to a type
in order to assign `&self` a lifetime.

Once again, we can implment Source on an owned type reliably, saving
the need for pointer indirections.