RFC: cmp and ops reform

[aturon]

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