fix tests
Deleted ListZipper' type-class, but kept MaybeListZipper data structure. It is argued that `type MaybeListZipper a = Maybe (ListZipper a)` is more appropriate. I am not so sure, but that might be a future revision. All the tests were fixed such that they now compile.

This is to fix issue #2.
merge upstream
fix Functor#fmap