More steps towards tracking of identifier scopes.

- provide scope to parse functions; if non-nil, parser uses the scope
  to declare and lookup identifiers
- resolve forward references where possible

R=rsc
CC=golang-dev
https://golang.org/cl/194098

src/cmd/gofmt/rewrite.go

@@ -37,7 +37,7 @@ func initRewrite() {
 // but there are problems with preserving formatting and also
 // with what a wildcard for a statement looks like.
 func parseExpr(s string, what string) ast.Expr {
-	x, err := parser.ParseExpr("input", s)
+	x, err := parser.ParseExpr("input", s, nil)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "parsing %s %s: %s\n", what, s, err)
 		os.Exit(2)