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/mpprint.c

@@ -207,7 +207,7 @@ int mp_print_mp_int(const mp_print_t *print, mp_obj_t x, int base, int base_char
     // If needed this function could be generalised to handle other values.
     assert(base == 2 || base == 8 || base == 10 || base == 16);
 
-    if (!MP_OBJ_IS_INT(x)) {
+    if (!mp_obj_is_int(x)) {
         // This will convert booleans to int, or raise an error for
         // non-integer types.
         x = MP_OBJ_NEW_SMALL_INT(mp_obj_get_int(x));