Is registering pointers to functions really necessary, or would defining
macros work as well? From a performance perspective I would like to
avoid having a pointer indirection step every time malloc/realloc/free
is called.

I guess my question becomes, Jukka, is this more for alternative
implementations of Python where changes to source are already expected,
or for apps that embed Python where a change of malloc/realloc/free
varies from app to app that dynamically loads Python?