Added fold method to solution of iterators5

[vallops99]

Another implementation of count_collection_iterator that could be useful for beginners in order to explore different ways of achieving the same thing.
This just avoids the "sum" method and directly sum each value returned from count_iterator with an accumulator

[mo8it]

This is how sum is implemented. fold is more powerful, but also harder to understand. Therefore I will not add it to this solution.