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'm not sure this convention is well followed in the present. Rc and Vec being two notable examples. This change in particular was suggested by @cmr.

It sort of makes sense, really. An entry is really just a *very smart* pointer to an element.