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)
@Valloric it would be difficult to remove `truncate` as you can't always manifest a `T` to call resize to truncate. C++ looks like it leverages overloading as well to provide some more functionality which we may not be able to match exactly.