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)
If one wants to ensure that a vector is exactly 10 items long, this is the best method to call since it will work right even if the vector is currently 9 or 11 items long.

IMO `grow` and `truncate` are the convenience methods, not `resize`.