Transaction building fixes

[SupernaviX]

All the fixes needed to build a transaction in my app!

• Add support for a "custom" ledger (anything which implements the Ledger trait). I'm using one powered by blockfrost.
• Hard-code a 2-ada fee. It would be better to compute the actual fee, but that looks very complicated and the logic for it probably should live somewhere more reusable.
• Fix change calculation. The old impl used a TotalLovelaceMinusFee output, which took a list of TXOs and returned their values minus the fee. It ignored any lovelace/assets which were spent as part of other TXs. This implementation uses a TotalChange output, which looks at the values of all other inputs and outputs to run its calculation.