* add -diff command line option
  * use scoping information in refersTo, isPkgDot, isPtrPkgDot.
  * add new scoping-based helpers countUses, rewriteUses, assignsTo, isTopName.
  * rename rewrite to walk, add walkBeforeAfter.
  * add toy typechecker, a placeholder for go/types

R=gri
CC=golang-dev
https://golang.org/cl/4285053