First pass at implementing VotingPool.Withdrawal

Still lots of TODOs/XXXs and just a single (and overly long) test, but I've
been working on this too long now; it's time to get some feedback and start
tackling the remaining things step by step.

Oh, and this depends on https://github.com/gsalgado/btcscript/tree/rename-signTxOutput, which has already been submitted (but not yet accepted) upstream.
This we can do - since we know which series have been thawed (or we can know it). But the withdrawal argument explicitly specifies the series for looking for inputs, so we should use that - also because we might have more than one hot series.