Collections Reform Part 2

This RFC shores up the finer details of collections reform. In particular, where the previous RFC
focused on general conventions and patterns, this RFC focuses on specific APIs. It also patches
up any errors that were found during implementation of part 1. Some of these changes
have already been implemented, and simply need to be ratified.

[rendered](https://github.com/Gankro/rfcs/blob/collections2/text/0000-collections-reform-part-2.md)
I don't feel it is. For one we expose a `with_b` constructor (though this will remain experimental). Otherwise I think it's relevant information to someone who actually cares about that sort of thing. Otherwise people will just see SomethingSomethingTreeMap and use it anyway.

And TreeMap may one day return.