giscanner: Correctly consume field annotations on structs

A hidden exception was being thrown (which we now log), due to fields
being treated as function parameters. Fixed to make field array
annotations be transformed and written out to the gir correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=710561
giscanner: Include field documentation

Structure field documentation was not being included in the gir file.

https://bugzilla.gnome.org/show_bug.cgi?id=710562