Replace AnyCurrency, CurrencyProtocol with new Currency implementation

Motivation:

The new implementation has been proven to work, and the old implementation is ready to be replaced.

Modifications:

- Add: Usage examples of Currency as an existential
- Change: ISO Currency type codegen to drop `_New_` prefix on new `Currency` conformances
- Remove: `AnyCurrency`, `CurrencyProtocol` protocols and the codegen conformances
- Remove: Unused `Decimal` extensions

Result:

The new `Currency` implementation is now the canonical representation in the library.