Make ints_of_string generic and move to lib.

lib/aoc.mli

@@ -1,6 +1,6 @@
-val ints_of_string : string -> int list
-(** [nums_from_string s] takes a string of space separated integers and gives
-    back a list of the integers. *)
+val ints_of_string : ?sep:string -> string -> int list
+(** [nums_from_string ?sep s] takes a string of integers separated by [sep] and
+    gives back a list of the integers.  By default [sep] is " " *)
 
 val distance1 : int -> int -> int
 (** [distance1 a b] returns the absolute difference between [a] and [b]. *)