Use internal iteration in the Sum and Product impls of Result and Option
This PR adds internal iteration to the `ResultShunt` iterator type underlying the `Sum` and `Product` impls of `Result`. I had to change `ResultShunt` to hold a mutable reference to an error instead, similar to `itertools::ProcessResults`, in order to be able to pass the `ResultShunt` itself by value (which is necessary for internal iteration).
`ResultShunt::process` can unfortunately no longer be an associated function because that would make it generic over the lifetime of the error reference, which wouldn't work, so I turned it into the free function `process_results`.
I removed the `OptionShunt` type and forwarded the `Sum` and `Product` impls of `Option` to their respective impls of `Result` instead, to avoid having to repeat the internal iteration logic.
c32735d03c