These are syntactical changes to better reflect the communication
operator's new status in the language.

- sending to a channel is now done via a send statement
- there is no binary communication operation anymore which
  leads to a reduction of the number of precedence levels
  from 6 to 5 (yeah!)
- small semantic change: since a send operation is not part
  of the expression syntax anymore, a <- send operator is
  binding weaker than any other operator now
- receiving from a channel is done as before via the unary
  receive expression
- communication clauses in select statement now can contain
  send statements or receive expressions

R=rsc, r, iant, ken2, gri1
CC=golang-dev
https://golang.org/cl/3973051