generate_packets.py: Move key field "unfill" into type code
I'm not completely happy with this patch (struct fields being passed by-reference to handle/dsend make reusing fill difficult here); but it works fine for all current use cases, and I'll have to go over all the fill stuff again anyway.
Part of #45206. Introduce a new Field.get_unfill() method, which calls a FieldType.get_code_unfill() method that stores field values in local variables, for use in Variant.get_delta_receive_body().