Simply return an reversed iterator from `__reversed__`.

Currently you're returning an instance of `FunctionalList` with `self.values` set as a `listreverseiterator` object. The problem with this is that user will think that they got an instance of `FunctionalList` back but when they will try to perform any of the operation like `len()`, `__getitem__`, `__setitem__` etc they will end up in errors. So, IMO it's better to return a simple `listreverseiterator` object.