Use more conventional syntax for constructors.

They are now invoked like "new Foo".
Also, superclass constructors are now much less semantically
and syntactically weird. Since the instance is created before
any constructor is called, there's no point in time where the
instance isn't there.

src/wren_vm.c

@@ -996,13 +996,9 @@ Value interpret(WrenVM* vm, Value function)
 
     CASE_CODE(NEW):
     {
-      // The current receiver is the class itself.
-      Value receiver = fiber->stack[frame->stackStart];
-      Value instance = wrenNewInstance(vm, AS_CLASS(receiver));
-
-      // Store the new instance back in the receiver slot so that now it can be
-      // "this" in the body of the constructor and returned by it.
-      fiber->stack[frame->stackStart] = instance;
+      // TODO: Handle object not being a class.
+      ObjClass* classObj = AS_CLASS(POP());
+      PUSH(wrenNewInstance(vm, classObj));
       DISPATCH();
     }