RFC: cmp and ops reform

This RFC proposes a number of design improvements to the `cmp` and
`ord` modules in preparation for 1.0. The impetus for these
improvements, besides the need for stabilization, is that we've added
several important language features (like multidispatch) that greatly
impact the design. Highlights:

* Make basic unary and binary operators work by value and use associated types.
* Generalize comparison operators to work across different types; drop `Equiv`.
* Refactor slice notation in favor of *range notation* so that special
  traits are no longer needed.
* Add `IndexSet` to better support maps.
* Clarify ownership semantics throughout.

[Rendered](https://github.com/aturon/rfcs/blob/cmp-ops-reform/text/0000-cmp-ops-reform.md)
Since `Deref` would have no input type parameter with this RFC, that means that `&*vec`/`&*string` would unambigiously produce `&[T]`/`&str`, so we'll end with two sugary ways to do the same thing. I think we should consider dropping this `[]`/`[..]` notation.