RFC: Collections reform

This is a combined *conventions* and *library stabilization* RFC. The goal is to establish a set of naming and signature conventions for `std::collections`.

The major components of the RFC include:

* Removing most of the traits in `collections`.

* A general proposal for solving the "equiv" problem, as well as improving `MaybeOwned`.

* Patterns for overloading on by-need values and predicates.

* Initial, forwards-compatible steps toward `Iterable`.

* A coherent set of API conventions across the full variety of collections.

*A big thank-you to @Gankro, who helped collect API information and worked through an initial pass of some of the proposals here.*

[Rendered](https://github.com/aturon/rfcs/blob/collections-conventions/active/0000-collections-conventions.md)
That's a good idea in any case.

I struggled with what to do with this API. Ideally, we would expose an iterator-based API, an immutable update API, *and* an in-place mutation API for each of these operations, since performance tradeoffs vary for each. But it's hard to find clear names for each of those categories simultaneously. I would love to hear any further suggestions.