made src a lib so that it loads into utop

also made an exe dir that does what `main.ml` used to do.
now `dune exec aoc` and `dune utop .` both do what they should :)

exe/main.ml

@@ -0,0 +1,6 @@
+open Aoc
+
+let () =
+  Day1.print ();
+  Day2.print ()
+;;