Add an unwinding Iterator that converts Iterator<Iterator<T>> into an Iterator<T>

There is a need for an iterator that is constructed from an Iterator<Iterator<T>> and unwinds the outer iterator to simply iterate over the inner Iterator<T>.

Unlike the IteratorChain<T> the inner iterators should not be constructed until needed so that any overhead involved in constructing an enclosed Iterator<T> is amortized across the entire operation and not front loaded on the constructor or first call.