Reviewed by maya@
Changes:
* marshal NULL strings as nil instead of empty strings. This allows use of e.g. DataInputStream:read_line() APIs.
* fix and improve build for OSX and Win-based configurations
* add support for arrays with lengths as struct fields
* allow GLib.Variant construction for lightuserdata
* fix gtop binding (certain structs could not be imported)
* adapt to new set of annotations in newer glib
* assorted Lua5.3 fixes, lgi is now fully Lua5.3 compatible
* fix binding of Gdk.Rectangle from newer GDK
768071387e