This change might be somewhat controversial, so I did it on a separate branch to make it easier to review and possibly reject. The main idea is that the balance/collateral wallet is now explicit, and so is the list of additional signers, if any.
This was a very useful exploration, and led to the design of the current TxSkel, which has a list of signing wallets, the first of which is the balancing wallet by default.
This change might be somewhat controversial, so I did it on a separate branch to make it easier to review and possibly reject. The main idea is that the balance/collateral wallet is now explicit, and so is the list of additional signers, if any.