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 wrote up a range type for rust-postgres here that's a bit more robust - with support for inclusive and exclusive bounds as well as the empty range. It's more than what's needed for this syntax, but if we're going to have the types, it might be worth making them more fleshed out: https://github.com/sfackler/rust-postgres/blob/master/src/types/range.rs