This change should have no user impact.
We commonly pair a currency, like USD, and an amount, like 1.23. This
adds the `FiatMoney` struct. It's a simple struct with two fields.
After adding it, I tried to use it everywhere. (It's possible I missed a
spot.)
I think this is a useful change on its own, but it'll be nice for an
upcoming change, too.
See also: [Android's equivalent class][0].
[0]: cb65347bb3/core-util/src/main/java/org/signal/core/util/money/FiatMoney.java (L1)
4675434c6c