Record our internal 'nullable' and 'optional' attributes into the
written .gir file.  It is now theoretically possible to express the
concept of an out parameter with a nullable type (although presently
there is no way to do this).

Modify our own internal parser (in the scanner) to understand the
newly-written attributes.

Update the expected output of the 'Regress-1.0.gir' test to account for
the new attributes.

Nothing else understands 'nullable' yet, but the girparser in the
typelib compiler already understands 'optional' and records a bit for it
in the typelib.

https://bugzilla.gnome.org/show_bug.cgi?id=660879