fix: ignore extra data in discv4 messages

[MegaRedHand]

Motivation

According to the spec:

Implementations should ignore any additional elements in the packet-data list as well as any extra data after the list.

We're currently failing if any of that happens.

Description

This PR introduces Decoder::finish_unchecked, which ignores any trailing bytes instead of failing, and uses that to allow for additional elements inside the packet-data. Also, it replaces some usage of RLPDecode::decode with RLPDecode::decode_unfinished to allow for extra data after the message.

As an extra, it refactors out the big match inside of both Message encode and decode functions, since it's expected those will change often in the next PRs.

Related to #17