Collateral selection improvements

[micahkendall]

Collateral selection can be improved variously

When ranking UTxOs, we should subtract the appropriate min-UTxO requirements due to non-Lovelace assets that will need to be returned as change.

We should also proceed with the largest ada utxo even if there are none below 5 Ada because you can have valid script transactions with quite low collateral, however we should throw errors in the late stages if that collateral utxo is not sufficient.

[joacohoyos]

@micahkendall i believe we fixed this in the last couple of iterations didn't we?