HDW: Add an AUTHENTICATION key purpose that's mapped to the internal chain.
HDUtils: less insistence on immutable params
Remove TransactionConfidence.workDone. It doesn't seem useful and was somehow buggy.
Introduce Fiat as a holder for fiat values. Coin and Fiat share an interface Monetary so that monetary infrastructure can be re-used.

Adapt CoinFormat to be used with Monetary. Add an ExchangeRate value class that can convert from Coin to Fiat and back.