Implement a multimap

[treeman]

Extensions to treemap and hashmap, with set variations. Indicated by the wiki.

[treeman]

#15623 as a start to a MultiSet.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#784