Use wrapper structs for `BTreeSet`'s iterators.

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 commit changes the keys and values iterators of `BTreeSet` 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].
Use wrapper structs for `HashSet`'s iterators.

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 commit changes the keys and values iterators of `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].