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 think this name is confusing. We already have `SlicePrelude::split_at[_mut]` which consumes itself and returns two copies of the same type. Shouldn't this be `pub fn split_at(self, at: uint) -> (Self, Self);` following the precedent?