Move monzo_ynab into //tools

Optimizing is difficult: I like flat hierarchies because I don't like
directory-hopping, but I also would like a cleaner root for my mono-repo. Bombs
away!

Well it's that time again, folks: spring cleaning!

Here I am musing about a few things that bother me:
- Should I use kebab-case or snake_case?
- It feels ~confusing to have //tools and //utils. What a //projects? Isn't
  everything a project? *sigh*

tools/monzo_ynab/shell.nix

@@ -0,0 +1,9 @@
+let
+  pkgs = import <nixpkgs> {};
+in pkgs.mkShell {
+  buildInputs = [
+    pkgs.go
+    pkgs.goimports
+    pkgs.godef
+  ];
+}