RFC: cmp and ops reform

This RFC proposes a number of design improvements to the `cmp` and
`ops` 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)
The latter: there's no change here regarding the relationship between `Eq` and `PartialEq`, etc. I wish we could do better, but doing so needs additional language features (as discussed in "Alternatives").