go spec: change def. of "type compatibility" to be non-recursive

go spec: change def. of "type compatibility" to be non-recursive
and adjust conversion rules.

Also:
- clarification of type identity (no language change)
- adjust special rules for channel assignment/comparison to
  require identical element types (in correspondence to non-
  recursiveness of type compatibility)

R=rsc, iant, ken2, r
CC=golang-dev
https://golang.org/cl/1376042