fix Zipper#foldLeft

previous version does not satisfy the Foldable law
override zipperInstance#foldMap. avoid stack overflow
enable Zipper traverse laws test
override ephemeralStreamInstance foldRight, foldMap
add infinite EphemeralStream foldRight, foldMap test
Merge pull request #529 from xuwei-k/Zipper-foldLeft

fix Zipper#foldLeft
Merge pull request #532 from xuwei-k/infinite-EphemeralStream-fold

override ephemeralStreamInstance foldRight, foldMap
remove code duplication