use foldLeft which is faster in Iterable Foldable

foldRight does a reverse which foldLeft does not need to do. Ran the tests and they pass. Not sure how to test individual laws though. The description here  http://eed3si9n.com/learning-scalaz/Combined+Pages.html#Functor+Laws now longer seems to work.
The document should tell more about this https://github.com/scalaz/scalaz/blob/series/7.2.x/CONTRIBUTING.md