We would like to return an error when doing math ops on amount types.
We cannot however use the stdlib `Result` or `Option` because we want to
implement ops on the result type.
Add an `AmountOpResult` type. Return this type from all math operations
on `Amount` and `SignedAmount`.
Implement `core::iter::Sum` for the new type to allow summing iterators
of amounts - somewhat ugly to use, see tests for example usage.
Add macros for implementing `ops::Add` and `ops::Sub` for various
combinations of references. Use the macro in `fee_rate`.
These are internal macros so no need to protect `core` using
`$crate::_export::_core`.
Tobin C. Harding
Fmt Bot