rollup merge of #19770: csouth3/iterator-wrapperstructs

Using a type alias for iterator implementations is fragile since this exposes the implementation to users of the iterator, and any changes could break existing code.

This PR changes the iterators of `BTreeMap`, `BTreeSet`, `HashMap`, and `HashSet` to use proper new types, rather than type aliases.  However, since it is fair-game to treat a type-alias as the aliased type, this is a:

[breaking-change].

diff --cc src/libcollections/btree/map.rs
Simple merge

diff --cc src/libstd/collections/hash/set.rs
Simple merge