Make AST_Class inherit AST_Scope instead of AST_Object

This is one of those days I'd love to use multiple inheritance.

An AST_Class has lots of common with AST_Object, but unfortunately
`instanceof AST_Scope` is used very, very much, and a class has its name
inside its own special pocket scope. This compels me to make AST_Class
inherit Scope instead.

It looks like, although there is much in common with AST_Object,
`instanceof AST_Object` seldom are made, perhaps because it is less
often necessary to traverse an object than a scope.

lib/output.js

@@ -502,12 +502,6 @@ function OutputStream(options) {
         return first_in_statement(output);
     });
 
-    // Not a class, though. It can be alone in a statement although
-    // it extends from AST_Object.
-    PARENS(AST_Class, function() {
-        return false;
-    });
-
     PARENS([ AST_Unary, AST_Undefined ], function(output){
         var p = output.parent();
         return p instanceof AST_PropAccess && p.expression === this;