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/compress.js

@@ -1245,7 +1245,7 @@ merge(Compressor.prototype, {
                 || this.alternative.has_side_effects(compressor);
         });
         def(AST_Unary, function(compressor){
-            return member(this.operator, ["delete", "++", "--", "yield", "yield*"])
+            return member(this.operator, ["delete", "++", "--"])
                 || this.expression.has_side_effects(compressor);
         });
         def(AST_SymbolRef, function(compressor){
@@ -2968,4 +2968,11 @@ merge(Compressor.prototype, {
         return self;
     });
 
+    OPT(AST_Yield, function(self, compressor){
+        if (!self.is_star && self.expression instanceof AST_Undefined) {
+            self.expression = null;
+        }
+        return self;
+    });
+
 })();