Flush stdout between each part.

This helps indicate where we have reached in case of problems.

lib/aoc.ml

@@ -12,7 +12,8 @@ let main prep parts =
     | [| _; fname |] ->
         let lines = prep fname in
         let do_part i (fmt, fn) =
-          Printf.printf "Part %d = %s\n" (i + 1) (fmt (fn lines))
+          Printf.printf "Part %d = %s\n" (i + 1) (fmt (fn lines));
+          flush stdout
         in
         List.iteri do_part parts;
         exit 0