Add block rewards

[liamzebedee]

This PR implements the block reward:

• tokenomic curve which mirrors bitcoin, documented in tokenomics_test.go and the python notebook.
• coinbase tx is mandatory now - every block must have at least 1 tx, the 1st tx is always coinbase, and the tx's amount must equal the output of GetBlockReward(block.height)
• state machine handles the coinbase mint now

This differs from testnet 1 in which:

• the block reward was static and unchanging
• the network didn't have decimals

Implements: #15