Cleanups:

- remove visitor pattern interface (not used)
- added non-exported "branding" methods to enforce
  node hierarchy

R=rsc
DELTA=174  (13 added, 92 deleted, 69 changed)
OCL=33838
CL=33963

src/pkg/go/parser/parser.go

@@ -1793,7 +1793,7 @@ func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction, getSemi
 	// convert vector
 	specs := make([]ast.Spec, list.Len());
 	for i := 0; i < list.Len(); i++ {
-		specs[i] = list.At(i);
+		specs[i] = list.At(i).(ast.Spec);
 	}
 
 	return &ast.GenDecl{doc, pos, keyword, lparen, specs, rparen}, gotSemi;