refactor: add lib.my for useful functions

lib/default.nix

@@ -0,0 +1,16 @@
+{ lib, ... }:
+
+lib.makeExtensible (self: rec {
+  mkDate = longDate: lib.concatStringsSep "-" [
+    (builtins.substring 0 4 longDate)
+    (builtins.substring 4 2 longDate)
+    (builtins.substring 6 2 longDate)
+  ];
+  mkVersionSrc = src: "unstable-" + builtins.substring 0 7 src.rev;
+  mkVersionInput = input:
+    let
+      date = mkDate (input.lastModifiedDate or "19700101");
+      rev = input.shortRev or "dirty";
+    in
+    "unstable-${date}_${rev}}";
+})