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)
This is something a few people has asked for. I don't have a strong desire/motivation for it, but I thought I would give it a fair try out in this RFC.

Asked for in https://github.com/rust-lang/rust/pull/19104#issuecomment-64085419

so maybe @nodakai can give a better justification.