Use some macros in native_internal (python#19757)

This should be a no-op from semantics/format point if view. Essentially
three things here:
* Add some macros to hide boilerplate for common steps
* Make use of some magic constants more obvious (most notably `2`)
* Add some `likely()/unlikely()` directives (this actually seems to have
some small positive effect).

Author: ilevkivskyi