Make function table read-only when done
Similarly to what's done in grilo-plugins: https://gitlab.gnome.org/GNOME/grilo-plugins/blob/master/src/lua-factory/grl-lua-library-operations.c#L734
Similarly to what's done in grilo-plugins: https://gitlab.gnome.org/GNOME/grilo-plugins/blob/master/src/lua-factory/grl-lua-library-operations.c#L734