WIP: implement EIP-5806 (type 4) delegate transaction

[Amxx]

This is a work-in-progress implementation of EIP-5806 (type 4) delegate transactions.

For references:

• this is the PR that implements EIP-5806 in geth.
• this is a script I've been using for testing.

TODO:

• [ ] Tests
• [ ] Documentation
• [ ] Changelog

[ricmoo]

This is similar to eip-2803? I’d love to see something like this championed. :)

[Amxx]

EIP-5806 is being actively developed. There are talks of a possible testnet, and inclusion in Prague.

We are still far from it, and it may not happen at all, but this PR is part of the "feasibility evaluation"