Change how new operators are compiled.

Instead of an instruction that always instantiates an instance,
the "new" is compiled to a method call on the class, followed by a
method call on the (presumably) new instance.

The former lets built-in types like Fn do special stuff instead of
allocating an instance. In particular, it means:

  new Fn { ... }

can just return the block argument.

src/wren_vm.h

@@ -150,9 +150,6 @@ typedef enum
   // stack.
   CODE_RETURN,
 
-  // Create a new instance of the receiver, which is assumed to be a class.
-  CODE_NEW,
-
   // Create a new list with [arg] elements. The top [arg] values on the stack
   // are the elements in forward order. Removes the elements and then pushes
   // the new list.