Complete challenge 18, implement CTR mode
Modularize solution 18
Add Encryptable typeclass

This allows me to make things ever more abstract.
First commit of work on challenge 19

This includes a fancy use of bind which I will probably rewrite to use
'do' instead.
Add some functions to assist in search

This is to help a user manually crack the ciphertexts.
Implement the score function from score.py

Pretty much the exact same algorithm