Implement tokenomics module

[liamzebedee]

Basically the coinbase is not validated at all right now. It will mint whatever the amount is contained in Amount.

It should be something like a bitcoin supply curve - ie. a pure function of height. GetBlockReward(height) -> uint

Logically speaking, we don't even need to embed the value of the coinbase transaction, since it's implicit in the protocol. However it might be useful for just simplicity reasons.