py: Downcase all MP_OBJ_IS_xxx macros to make a more consistent C API.

These macros could in principle be (inline) functions so it makes sense to
have them lower case, to match the other C API functions.

The remaining macros that are upper case are:
- MP_OBJ_TO_PTR, MP_OBJ_FROM_PTR
- MP_OBJ_NEW_SMALL_INT, MP_OBJ_SMALL_INT_VALUE
- MP_OBJ_NEW_QSTR, MP_OBJ_QSTR_VALUE
- MP_OBJ_FUN_MAKE_SIG
- MP_DECLARE_CONST_xxx
- MP_DEFINE_CONST_xxx

These must remain macros because they are used when defining const data (at
least, MP_OBJ_NEW_SMALL_INT is so it makes sense to have
MP_OBJ_SMALL_INT_VALUE also a macro).

For those macros that have been made lower case, compatibility macros are
provided for the old names so that users do not need to change their code
immediately.

py/objenumerate.c

@@ -78,7 +78,7 @@ const mp_obj_type_t mp_type_enumerate = {
 };
 
 STATIC mp_obj_t enumerate_iternext(mp_obj_t self_in) {
-    assert(MP_OBJ_IS_TYPE(self_in, &mp_type_enumerate));
+    assert(mp_obj_is_type(self_in, &mp_type_enumerate));
     mp_obj_enumerate_t *self = MP_OBJ_TO_PTR(self_in);
     mp_obj_t next = mp_iternext(self->iter);
     if (next == MP_OBJ_STOP_ITERATION) {