Determine BTC encoding of Acceptor metadata

[MaddieM4]

The encoding question was a lot simpler when the only action available was "reference a quorum, thus binding it to a specific block". Now we also need to express "vote of confidence" and "vote of no confidence" with a Bitcoin address as an additional parameter.

One of the particular concerns here is expense - we want to make this efficient, possibly supporting batching of multiple actions into a single transaction. We also need to account for BTC's limitations to discourage "dust" transactions, which may provide some hard lower limits on monetary sums.

[MaddieM4]

Part of #47.