Improve yield support and restrict usage of strict

- Partially reverting 91cdb93e57 and eaf3911c31 and reimplement
- Add generators support for objects and classes
- Only classes can have static methods so restrict use of it

Special thanks to @rvanvelzen and @kzc for reviewing this patch and
providing constructive feedback over and over again.

lib/scope.js

@@ -575,6 +575,8 @@ AST_Toplevel.DEFMETHOD("compute_char_frequency", function(options){
             base54.consider("catch");
         else if (node instanceof AST_Finally)
             base54.consider("finally");
+        else if (node instanceof AST_Yield)
+            base54.consider("yield");
         else if (node instanceof AST_Symbol && node.unmangleable(options))
             base54.consider(node.name);
         else if (node instanceof AST_Unary || node instanceof AST_Binary)