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.
Amount add and sub now enforce the MAX_MONEY invariant when doing
addition and subtraction. We need to tell kani to assume we don't
overflow before doing actual tests.
Note also that `ops::Add` calls through to `checked_add` and
`ops::Sub` calls through to `checked_sub` so separate kani tests for
these are unnecessary.
Tobin C. Harding
Fmt Bot