Move IntPair and PosSet into Aoc library.

These are obviously generic enough and useful for the future that they
can be moved to a shared library.

lib/aoc.ml

@@ -23,3 +23,12 @@ let main prep parts =
   with e ->
     Printf.printf "An error occured: %s\n" (Printexc.to_string e);
     exit 1
+
+module IntPair = struct
+  type t = int * int
+
+  let compare (x, y) (x', y') =
+    match compare y y' with 0 -> compare x x' | c -> c
+end
+
+module IntPairSet = Set.Make (IntPair)