py/objint: from_bytes(): Implement "byteorder" param and arbitrary precision.

If result guaranteedly fits in a small int, it is handled in objint.c.
Otherwise, it is delegated to mp_obj_int_from_bytes_impl(), which should
be implemented by individual objint_*.c, similar to
mp_obj_int_to_bytes_impl().

py/objint_mpz.c

@@ -107,6 +107,12 @@ char *mp_obj_int_formatted_impl(char **buf, size_t *buf_size, size_t *fmt_size,
     return str;
 }
 
+mp_obj_t mp_obj_int_from_bytes_impl(bool big_endian, size_t len, const byte *buf) {
+    mp_obj_int_t *o = mp_obj_int_new_mpz();
+    mpz_set_from_bytes(&o->mpz, big_endian, len, buf);
+    return MP_OBJ_FROM_PTR(o);
+}
+
 void mp_obj_int_to_bytes_impl(mp_obj_t self_in, bool big_endian, size_t len, byte *buf) {
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_int));
     mp_obj_int_t *self = MP_OBJ_TO_PTR(self_in);