py: Change mp_const_* objects to macros.

Addresses issue #388.

py/objnone.c

@@ -22,12 +22,11 @@ STATIC mp_obj_t none_unary_op(int op, mp_obj_t o_in) {
     }
 }
 
-const mp_obj_type_t none_type = {
+const mp_obj_type_t mp_type_NoneType = {
     { &mp_type_type },
     .name = MP_QSTR_NoneType,
     .print = none_print,
     .unary_op = none_unary_op,
 };
 
-STATIC const mp_obj_none_t none_obj = {{&none_type}};
-const mp_obj_t mp_const_none = (mp_obj_t)&none_obj;
+const mp_obj_none_t mp_const_none_obj = {{&mp_type_NoneType}};