caps/eth.md: specify encoding of all consensus objects (#177)

This adds a lot of new text to cover the encoding and validity of
consensus objects. We previously referred to the 'main Ethereum
specification' for that, but no such specification exists in practice.

There is also a new entry in the changelog to cover the addition of
EIP-2718 typed transactions in the wire protocol. While there is no
version bump.

Author: fjl