godoc, exp/ebnf, exp/types, go/scanner, scanner: use rune

godoc, exp/ebnf, exp/types, go/scanner, scanner: use rune

API question: is a scanner token an int or a rune?

Since the rune is the common case and the token values
are the special (negative) case, I chose rune.  But it could
easily go the other way.

R=gri
CC=golang-dev
https://golang.org/cl/5301049