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.

test/parser.js

@@ -128,7 +128,7 @@ module.exports = function () {
     });
 
     var generators_yield_def = UglifyJS.parse('function* fn() {\nyield remote();\}').body[0].body[0];
-    ok.equal(generators_yield_def.body.operator, 'yield');
+    ok.strictEqual(generators_yield_def.body.is_star, false);
 }
 
 // Run standalone