for certain reports.
Those reports being ones using complex options, apparently because
the callbacks weren't protected from Guile's garbage collector.
So replace the anyway ugly hack of a void* with a std::any wrapping
a class holding a std::unique_ptr with a custom deleter. The
constructor calls scm_gc_protect_object on the SCM containing the
callback and the custom deleter calls scm_gc_unprotect_object. The
copy constructor, required for std::any, makes a new std::unique_ptr
and calls scm_gc_protect_object again ensuring that the protect and
unprotect calls are symmetrical.
Meanwhile std::any hides the Guile dependency from all the classes
that don't need to know about it. The only ugliness is that there's
no good place to put a common implementation of SCNCallbackWrapper so it's
repeated in gnc-optiondb.i and dialog-options.cpp.
