py: Change mp_const_* objects to macros.

Addresses issue #388.
diff --git a/py/vm.c b/py/vm.c
index 1a1d345..1f349de 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -173,7 +173,7 @@
                         break;
 
                     case MP_BC_LOAD_CONST_ELLIPSIS:
-                        PUSH(mp_const_ellipsis);
+                        PUSH((mp_obj_t)&mp_const_ellipsis_obj);
                         break;
 
                     case MP_BC_LOAD_CONST_SMALL_INT: {