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/dune

@@ -0,0 +1,4 @@
+(executable
+ (public_name aoc)
+ (name main)
+ (libraries aoc))

exe/main.ml

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

src/dune

@@ -1,5 +1,4 @@
-(executable
- (public_name aoc)
- (name main)
+(library
+ (name aoc)
  (libraries stdio re)
  (preprocess (pps ppx_regexp)))

src/main.ml

@@ -1,4 +0,0 @@
-let () =
-  Day1.print;
-  Day2.print
-;;