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/objexcept.c

@@ -123,7 +123,7 @@ void mp_obj_exception_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kin
         } else if (o->args->len == 1) {
             #if MICROPY_PY_UERRNO
             // try to provide a nice OSError error message
-            if (o->base.type == &mp_type_OSError && MP_OBJ_IS_SMALL_INT(o->args->items[0])) {
+            if (o->base.type == &mp_type_OSError && mp_obj_is_small_int(o->args->items[0])) {
                 qstr qst = mp_errno_to_str(o->args->items[0]);
                 if (qst != MP_QSTR_NULL) {
                     mp_printf(print, "[Errno " INT_FMT "] %q", MP_OBJ_SMALL_INT_VALUE(o->args->items[0]), qst);
@@ -447,7 +447,7 @@ mp_obj_t mp_obj_new_exception_msg_varg(const mp_obj_type_t *exc_type, const char
 
 // return true if the given object is an exception type
 bool mp_obj_is_exception_type(mp_obj_t self_in) {
-    if (MP_OBJ_IS_TYPE(self_in, &mp_type_type)) {
+    if (mp_obj_is_type(self_in, &mp_type_type)) {
         // optimisation when self_in is a builtin exception
         mp_obj_type_t *self = MP_OBJ_TO_PTR(self_in);
         if (self->make_new == mp_obj_exception_make_new) {