go spec: clarify semantics of range clause

This CL proposes some subtle language changes
in an attempt to clarify the semantics of range
clauses and simplify uses of maps.

- nil maps behave like empty maps; but attempting
  to set a value in a nil map causes a run-time panic
- nil channels are never ready for communication;
  sending or reading from a nil channel blocks forever
- if there is only one index iteration variable in a
  range clause and len(range expression) would be a constant,
  the range expression is not evaluated.
  (was discrepancy with len/cap before)
- the notion of what is a constant expression len(x)
  for (pointer to) arrays x has been generalized and
  simplified (can still be syntactically decided)
  (before: more restrictive syntactic rule that was not
  consistently implemented)

Fixes #1713.

R=r, rsc, iant, ken2, r2, bradfitz, rog
CC=golang-dev
https://golang.org/cl/4444050