Optimize unfold/unfoldLeft for Lists

[tvelka]

At the moment the List::unfold and List::unfoldLeft are implemented as Iteartor.unfoldLeft(...).toList() which works but perhaps a better implementation would be

return Iterator.unfoldRight(seed, f.andThen(tupleOpt -> tupleOpt.map(Tuple2::swap)))
               .foldLeft(List.empty(), List::prepend);

This builds the result without any additional reverse() operations and Streams in a single for-loop.

[danieldietrich]

Hi, improving the implementations is always welcome! Are you volunteering for a PR?