Collect test vectors from the network for encoding / decoding

[Rjected]

There should be plenty of test vectors for correct encoding / decoding of real network messages, to make sure the RlpEncodable and RlpDecodable derives work correctly, as well as any custom Encodable and Decodable implementations.

To do this, we should collect encodings from the network and attempt to decode them. We should also check that the correct byte strings are output, given known decoded messages.

[Rjected]

Closing since this has been done for a lot of messages, but more test vectors are always appreciated (especially failing test vectors).