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 :)

src/dune

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