[refactor] encapsulate a unit's denomination in a class

Summary:
a class that represents a coin's denomination (unit, subunit and decimals of precision) rather than two constants and a magic number that's scattered around a few functions

does not change behavior

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9656