compiler: girparser: parse 'nullable' attribute

Parse the 'nullable' attribute on parameters and function return types.

Additionally, tweak the meaning of the 'allow-none' attribute.  We now
only treat it as equivalent to 'nullable' for non-out parameters.  For
out parameters, we treat it to mean the same as the already-recognised
'optional' parameter (which we only recently started actually using).

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