Add a generic main function.

lib/aoc.mli

@@ -8,3 +8,11 @@ val distance1 : int -> int -> int
 val strings_from_file : string -> string list
 (** [strings_from_file fname] returns a list of strings from the file 
     [fname].  Each string represents a line from the file. *)
+
+val main : (string -> 'a) -> (('b -> string) * ('a -> 'b)) list -> unit
+(** [main prep parts] executes an advent of code problem.  [prep fname] should
+    be a function that returns the input from [fname].  Each elemet of
+    [parts] is a pair of functions.  The first converts the output to a string
+    (for example [string_of_int]).  The second executes the given part.
+    Output is given as if done by: 
+    [print_string ( prep fname |> snd |> fst )] *)