Prefer snake-shift instead of a row-by-row shift

Per the assignment's instructions, the `Shift n` operation should treat
the *entire keyboard* like a cycle and shift that. I was erroneously
treating *each row* like a cycle and shifting those one-by-one.

This change fixes that. In addition, it also:
- Updates README.md with expected inputs and outputs
- Updates test suite
- Adds `split` dependency to {default,shell}.nix

scratch/brilliant/shell.nix

@@ -10,6 +10,7 @@ in pkgs.mkShell {
       hspec
       optparse-applicative
       unordered-containers
+      split
     ]))
   ];
 }