Only define functions for which the user provided a renaming #define
(instead of forcing the users to create name for everything)
Only define iterator struct if a rename #define is present (instead of
forcing the user to define NO_ITERATOR if he doesn't want one)

also flip true/false in the implementation to match the documentation in
irnodeset.h

Add wrapper macros for pset_first() and pset_next(), which have the return type as additional parameter.

Add wrapper macros for set_first() and set_next(), which have the return type as additional parameter.