This allows building the sighash preimages required for signing inputs.
We use UnsignedTx to precompute the hashes required for building the preimages, in case we need to sign a lot of inputs.
Only BIP143 sighash preimages are implemented, but not LEGACY; they aren't available on eCash since UAHF. Implementing it might be useful if we want to allow people to use ecash-lib on Dogecoin.