This field is set based on o_type when the object is created and it is
not changed during the lifetime of the object. Therefore we can check
o_type to choose the proper delete function in json_object_put(), and
save sizeof(void *) bytes in struct json_object_private.
- there is no need for _ref_count to be uint_fast32_t (the compiler
might decide to use a 64-bit int). make it uint32_t instead.
- reorder the 32-bit integer fields (o_type and _ref_count) so that
there is no wasted 4-byte gap after each of them.