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/default.nix

@@ -5,9 +5,10 @@ let
     rev = "afa9ca61924f05aacfe495a7ad0fd84709d236cc";
   }) {};
 
-  ghc = pkgs.haskellPackages.ghcWithPackages (hpkgs: [
-    hpkgs.optparse-applicative
-    hpkgs.unordered-containers
+  ghc = pkgs.haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [
+    optparse-applicative
+    unordered-containers
+    split
   ]);
 in pkgs.stdenv.mkDerivation {
   name = "transform-keyboard";