Allow usage with summing iterators

[eutampieri]

It would be useful if you implemented the Sum trait in order to use the .sum() on an iterator over a vec of fractions.
At the moment it fails with the following error:

the trait bound `fraction::fraction::GenericFraction<u64>: std::iter::Sum` is not satisfied
the trait `std::iter::Sum` is not implemented for `fraction::fraction::GenericFraction<u64>`rustc(E0277)

[eutampieri]

I've just added this to fraction/mod.rs and it works

impl<T: Clone + Integer> Sum for GenericFraction<T> {
    fn sum<I: Iterator<Item=Self>>(iter: I) -> Self {
        iter.fold(GenericFraction::<T>::neg_zero(), Add::add)
    }
}

[dnsl48]

Hi @eutampieri, thank you for the idea. Fair enough.

I think that would also make sense to add Product implementation.
Would you like to contribute a PR? Ideally that would be an implementation of both traits for Fraction and Decimal and some test coverage for them.
If you don't have time, I'll prepare a patch for the next minor release myself, no pressure :)