R1:f86f9dc51283

Make Currency a more formal type

Summary:
Ref T2787. Phortune currently stores a bunch of stuff as `...inUSDCents`. This ends up being pretty cumbersome and I worry it will create a huge headache down the road (and possibly not that far off if we do Coinbase/Bitcoin soon). Even now, it's more of a pain than I figured it would be.

Instead:

- Provide an application-level serialization mechanism.
- Provide currency serialization.
- Store currency in an abstract way (currently, as "1.23 USD") that can…