impl std::iter::{Sum, Product} for Ratio

[Emerentius]

Fixes #3

I only implemented Sum and Product for Ratio<T> and &Ratio<T> rather than any U, where Ratio<T>: Mul<U, Output = Ratio<T>>. Summing for example an iter of T directly into a Ratio<T> does not seem like particularly useful or critical functionality.

[cuviper]

Thanks! How about some quick test cases?

Summing for example an iter of T directly into a Ratio does not seem like particularly useful or critical functionality.

I agree. Sums and products of integers can be done directly first.

[Emerentius]

I've added a generic function each to for both sum and product as a type-level assert of applicability and one test computation as a sanity test for the correct result. I can add more samples, but I think I'd be testing Mul and Add more than sum and product.

[cuviper]

Yeah, that level of testing looks like plenty, thanks.

bors r+

[bors]

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