Specifically:
- Simplified definition of comparison compatibility and folded into
  section on comparison operators since it's only used there.

This is a small language change/cleanup. As a consequence:
- An interface value may now be compared against a non-interface value.
- Channels with opposite directions cannot be compared directly anymore
  (per discussion with rsc).

R=rsc, r, iant, ken2
CC=golang-dev
https://golang.org/cl/1462041