Add crypto-square exercise

The code is supposed to take the first character from a each set, rotating the sets round-robin. That made me first think like this:

[[Character | Rest_of_row] | Rest_of_rows],
Rest_of_rows ++ [Rest_of_row].

But taking something from the front of a list and putting it at the end of the same list, is just so very queue. Would better names, or comments, make the queue seem more natural to you.