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)
Most things that care about ExactSize also care about DoubleEnded, so I'm not sure if that's *totally* helpful. I agree that it's a borderline abuse to impl DoubleEnded on an "unordered" sequence, though.