Add some helpers to impldement %{deps[i]}

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/stdune/int.ml

@@ -13,3 +13,9 @@ include T
 
 module Set = Set.Make(T)
 module Map = Map.Make(T)
+
+let of_string_exn s =
+  match int_of_string s with
+  | exception Failure _ ->
+    failwith (Printf.sprintf "of_string_exn: invalid int %S" s)
+  | s -> s

src/stdune/int.mli

@@ -3,3 +3,5 @@ val compare : t -> t -> Ordering.t
 
 module Set : Set.S with type elt = t
 module Map : Map.S with type key = t
+
+val of_string_exn : string -> t

src/stdune/list.ml

@@ -104,3 +104,9 @@ let rec assoc t x =
   | (k, v) :: t -> if x = k then Some v else assoc t x
 
 let singleton x = [x]
+
+let rec nth t i =
+  match t, i with
+  | [], _ -> None
+  | x :: _, 0 -> Some x
+  | _ :: xs, i -> nth xs (i - 1)

src/stdune/list.mli

@@ -40,3 +40,5 @@ val compare : 'a t -> 'a t -> compare:('a -> 'a -> Ordering.t) -> Ordering.t
 val assoc : ('a * 'b) t -> 'a -> 'b option
 
 val singleton : 'a -> 'a t
+
+val nth : 'a t -> int -> 'a option