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)
clarification request: Are you saying that the changed design is *more* painful in this respect?  Or are you saying that even the prior design suffered from a similar amount of pain when it came to deriving `Eq`/`Ord` (which I would assume is because one needs to also implement/derive `PartialOrd` as well, as implied by the next sentence)?