The GType is fetchable from the GIRegisteredTypeInfo, and it is the only
member keeping the GjsArgumentCache struct from being smaller. However,
this is a bit of a tradeoff since g_registered_type_info_get_g_type()
involves a call to g_module_symbol().

Hopefully this doesn't have much of an effect on performance, compared
to the net benefit of introducing the argument cache in the first place,
since g_registered_type_info_get_type() is also used in the "old way" of
marshalling values in arg.cpp.