Fiat, Monetary and ExchangeRate

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.
I decided against this class because I think we will be limited to "official" currencies in this case. As I said earlier, exchange rate providers also provide altcoin rates.