pathencode: implement both basic and hashed encoding in C

pathencode: implement hashed encoding in C This will be used by an upcoming patch.

pathencode: implement the "mangling" part of hashed encoding in C This will be used by an upcoming patch.

pathencode: add a SHA-1 hash function This will be used by an upcoming patch. This calls out to the Python hash implementation. An earlier version of this function implemented SHA-1 directly, but the amount of extra code didn't seem like a good tradeoff compared to the small big-picture increase in performance (long paths are uncommon).