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.
Sure we can change this constant, and I already planned to. I would change it to 4 -- in the finance world its common to use sub-cent precision.