Mempool

[rustielin]

Added mempool functionality for a demo in a Bitcoin mechanics workshop I taught:

• dotx now creates a transaction, which then gets added to the mempool
• mempool lists the transactions currently in the mempool
• mineblocks groups transactions from the mempool into a block, and uses PoW to mine and append the block to the blockchain. By default, it completely clears the mempool, but there are optional index parameters, which specify transaction indices from the mempool

(BTW awesome project @daxeel! My students enjoyed this hands on activity a lot)

[daxeel]

Hi @rustielin, Fantastic work. Looking into your commits and running tests. Thanks for your awesome feedback.