Allow coinbases to interact with contracts with data field

[mibuono]

To support liquid mining tokens, we must allow inbound coinbase ETXs to interact with smart contracts. This will come in the form of a function signature in the data field of the ETX. If a coinbase ETX has a 'to' address that is a contract (has code) but there is no function signature in the data field (or if the function execution fails) the coinbase is lost. In addition, if the miner puts some data in the data field but the 'to' address is not a contract, the coinbase is also lost. Therefore it is up to the miners to put correct data in their coinbases.
If a miner puts a function signature in the data field and the 'to' address is a contract, the average gas price of the last n blocks will be chosen and multiplied by some gas limit, and then deducted from the coinbase value to pay for execution.
The miner of a block can modify their coinbase ETX to include data and a gas limit, but currently a workshare miner cannot. This may require modifying work objects to include data and gas limits.
The lockup byte will have to move to a field in the ETX.