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)
>However, in general it's a pretty dangerous operation to implement Deref because it means we'll never be able to backwards-compatible add a method to an OccupiedEntry (or Rc for that matter).

Adding public methods is always backwards incompatible because of extension traits.