Currently units::amount::Denomination does not implement PartialOrd or Ord. This prevents any type that includes a Denomination from being used in contexts that require these traits (e.g. as key to a BTreeMap). This is an unnecessary restriction. We should either implement the traits or implement ArbitraryOrd.

This raises the question - "should stable crates derive PartialOrd and Ord on enums"? Doing so locks us in to not being able to re-order the variants (which would otherwise be an internal refactor) because the derived impls use the enum integer value (0 for the first variant, 1 for the second etc).

Since we have ArbitraryOrd should we be using it for all enums? Or perhaps should we be manually implementing PartialOrd and Ord?