UDP-ard computes CRC16 Header _before_ modifying members (should be after)

[emrainey]

https://github.com/OpenCyphal-Garage/libudpard/blob/main/libudpard/udpard.c#L344

The CRC16 has to be computed AFTER all the members have been set.

Then it has to be byte swapped!

[pavel-kirienko]

Tightly related #13