Move memoize to the Aoc library.

lib/aoc.mli

@@ -30,6 +30,11 @@ val main : (string -> 'a) -> (('b -> string) * ('a -> 'b)) list -> unit
     [string_of_int]). The second executes the given part. Output is given as if
     done by: [print_string ( prep fname |> snd |> fst )] *)
 
+val memoize : ('a, 'b) Hashtbl.t -> ('a -> 'b) -> 'a -> 'b
+(** [memoize memo f value] returns the result of [f value]. The hashtable [memo]
+    is used to cache results, so repeated calls with the same [value] will not
+    call [f] again. *)
+
 (** Module representing a pair of integers, useful for Set.Make *)
 module IntPair : sig
   type t = int * int