Support for hard-fork contract

[agaffney]

There's a TUNAv2 contract with some changes. We want to be able to support both.

I've looked through the TUNA hard-fork contract, and I've noticed a few differences:

• the TargetState object (used by the miner internally) has its fields reordered and a new miner bytestring field added
• the miner bytestring is a blake2b hash of either a public key hash or a NFT (for mining pools, I think)
• if using a PKH for the miner field, the contract will verify that it matches one of the signers on the transaction
• the on-chain datum fields have been reordered to put extra (message) last instead of before the interlink

It's harder to determine what changes we'll need in the TX building (extra redeemers or whatever) from looking at the smart contract, so we may need to wait until Fortuna is updated to see what's needed

[agaffney]

This was done partially in #98

[agaffney]

This was finished in #192