This means we're not abusing the notifiers for meta_marshallres,
and we're able to later cleanly add other fields to GClosure.
We still have to leave the ABI intact for the GClosure->meta_marshal
bit, as old G_CLOSURE_N_NOTIFIERS macro instances still accesses it.
However, we always set it to zero to keep those macros working.
https://bugzilla.gnome.org/show_bug.cgi?id=661140
This way we don't need to keep a custom array that we bsearch on (and
that isn't threadsafe) but can use the gtype.c machinery that is
threadsafe. And fast, too!
https://bugzilla.gnome.org/show_bug.cgi?id=554887