Block header spec

[adlerjohn]

Fields:

1. MMR of previous block header hashes (binary Merkle tree is an MMR)
2. Coinbase address
3. Fuel block height
4. Ethereum block number (!) (current, not finalized)
5. timestamp (!) (current)
6. Transactions root (binary Merkle tree, not MST)
7. number of txs
8. next validator set root
9. next voting power
10. Hash of Tendermint commit for previous block (>2/3 of voting power) (canonical commit, client needs to broadcast this with proposal)
11. root of withdrawal IDs

Separate from block header:

1. Tendermint Commit for current block (verified by the contract)

Note: consider wrapping core, consensus, and Ethereum specific fields