packet_size may or may not include payload, depending on type

[mkeeter]

Calling packet_size on a UDP packet always returns 8, the header size. However, on an IPv6 packet, it returns a length that's inclusive of the payload. This is a surprising discrepancy!

I suspect this is because struct Ipv6 includes a #[length = "payload_length"] annotation, which is missing from struct Udp.

Would it make sense to add this annotation to struct Udp? If so, I'd be happy to open a PR.

Related: #169, #519; it's unclear which behavior is actually "correct" here.