Check for leftover white space

Agda stdlib team recommends to run
[fix-whitespace](https://github.com/agda/agda-stdlib/blob/master/HACKING.md#how-to-enforce-whitespace-policies)
in order to get rid of unnecessary white space characters.  I've added
this tool to the nix dev shell, as well as formed a check that will be
can be run with `nix flake check` command.  As of now this is entirely
opt-in as this check is not enabled in (and thus won't fail) the CI.

fix-whitespace.yaml

@@ -0,0 +1,13 @@
+included-dirs:
+  - src
+
+included-files:
+  - "*.agda"
+  - "*.md"
+  - "*.org"
+  - "*.nix"
+
+excluded-dirs:
+  - .git
+  - .direnv
+  - result

flake.nix

@@ -21,10 +21,22 @@
         agdaWithStandardLibrary = pkgs.agda.withPackages (_: [ standard-library ]);
 
       in {
-        devShell = pkgs.mkShell {
+        checks.whitespace = pkgs.stdenvNoCC.mkDerivation {
+          name = "check-whitespace";
+          dontBuild = true;
+          src = ./.;
+          doCheck = true;
+          checkPhase = ''
+            ${pkgs.haskellPackages.fix-whitespace}/bin/fix-whitespace --check
+          '';
+          installPhase = ''mkdir "$out"'';
+        };
+
+        devShells.default = pkgs.mkShell {
           buildInputs = [
             agdaWithStandardLibrary
             pkgs.graphviz
+            pkgs.haskellPackages.fix-whitespace
           ];
         };