- GEnum type for G_PRIORITY_VALUES?
- can't implement g_idle_remove_by_data because ... well, how would you
  search for the data value?
- proper cleanup for bad parameter types in Glib::Object::new?
- GFlags type for GSignalFlags
- gperl_sv_from_value and gperl_value_from_sv should croak on the unknown
  or unimplemented data type, because we should have an implementation for
  all the fundamentals.
  - may require the ability to register handlers for new fundamental types.
  - gperl_value_from_sv should change to void return if it croaks. 
