Created attachment 170573 [details] [review]
Correctly store and look up custom signal marshallers

Rather than storing (signal name => (marshaller, instance type))
associations, use (instance type => (signal name => marshaller)).  This
ensures that we handle signal names occurring more than once in the
object hierarchy.  For example, Gtk2::Dialog and Gtk2::InfoBar both have
a "response" signal (for which we want to install custom signal
marshallers).  This implies that the look up implementation used during
signal connection needs to walk the ancestry and the interfaces of the
instance's type in the search for custom signal marshallers.