go/ast: reflect communication operator changes accurately in ast

go/ast: reflect communication operator changes accurately in ast

- go/ast: introduce SendStmt; adjust SelectStmt
- go/parser: accept new communication syntax, minor
  unrelated cleanups
- go/printer: adjustments for new ast, fewer binary
  expression precedences
- go/token: remove one binary precedence

Adjusted dependent code. gofmt -w src -misc. Ran all tests.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/3989056