New message signing API

[carver]

What was wrong?

Message signing should be very pluggable according to EIP191. Message encoding is all built-in to the (now confusingly named) defunct_hash_message().

How can it be fixed?

New message signing API that captures this concept:

class SignableMessage(NamedTuple):  # as defined by EIP-191
  version: HexBytes  # must be length 1
  header: HexBytes  # aka "version specific data"
  body: HexBytes  # aka "data to sign"

signable_message = encode_structured_data_message(json_str)
account.sign_message(signable_message)

The encoding method could have arbitrary inputs without changing the message signing API, like:

signable_message = encode_intended_validator(message, validator_address)
account.sign_message(signable_message)

It abstracts away the EIP-191 encoding process and message hashing inside sign_message(). A new recover_message() could work similarly.

The solution should make it trivial to experiment with or produce your own (sub)version of EIP-191, without any change to the eth-account library. (This is already possible by signing and recovering hashes of your own making, but it's not as accessible).