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/tokens.nix

@@ -0,0 +1,23 @@
+{ depot, briefcase, ... }:
+
+let
+  auth = depot.buildGo.package {
+    name = "auth";
+    srcs = [
+      ./auth.go
+    ];
+    deps = with briefcase.gopkgs; [
+      utils
+    ];
+  };
+in depot.buildGo.program {
+  name = "token-server";
+  srcs = [
+    ./tokens.go
+  ];
+  deps = with briefcase.gopkgs; [
+    kv
+    utils
+    auth
+  ];
+}