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)
FWIW, I think the most surprising aspect of this can be solved via `#[must_use]` attributes on the 'set adaptor' types, similar to what is done [for the main iterator adaptors](https://github.com/rust-lang/rust/blob/5d335c94bd9017f16ce7538a5f4063e1178f15a1/src/libcore/iter.rs#L744).