Optional change purification

[vsubhuman]

1. Added a new flag field prefer_pure_change to the tx-builder, default set to false, but can be set true via a new setter
2. When calculating dirty change with some assets - only in case the flag is true - added a check if the remaining pure value of Coin (after adding asset outputs) is greater than the minimal required utxo value plus additional fee for adding a new output, if so - the pure ADA is added as a separate change output.

The change purification is made optional for cases when someone might have had any kind of business logic dependent on the file that some input marker tokens would always go back along with the main change Coin. So enabling the purification like that should be an explicit decision in any user project.

Added test cases for scenarios:

1. Dirty change has enough Coin so it gets split into another output
2. Dirty change has more Coin than minimal utxo value but it's not enough to cover additional fees, so no purification

[pedromtcosta]

LGTM!