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)
I'd rather see a type error than a runtime panic, in every instance possible. If `..3` isn't logically meaningful then let that be a type error. That's better for correctness and for performance.