go spec: follow-up cleanups after communication operator changes
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