Patches 1 & 2 are rather complex, but the end result is that we pass
closures + user_data + free function in single struct parameters as I
described previously in this email:
https://www.redhat.com/archives/libguestfs/2019-August/msg00210.html
Patch 3 adds a convenient FREE_CALLBACK macro which seems a worthwhile
simplification if you buy into 1 & 2.
Patch 4 adds another macro which is less obviously useful.
Rich.