permutation.py: Implemented `next` and `prev` methods to correspond to
Permutation.cpp's ++ and --, along with `firstOfDegree`
Permutation.hs and permutation.py, like Permutation.cpp, now sort their objects
in the same order as the modified Lehmer codes do.
Permutation.hs: Implemented `next`, `prev`, and `firstOfDegree` in Haskell