Add List.physically_equal

Signed-off-by: Etienne Millon <me@emillon.org>

src/lib.ml

@@ -533,7 +533,7 @@ module Dep_stack = struct
   let to_required_by t ~stop_at =
     let stop_at = stop_at.stack in
     let rec loop acc l =
-      if l == stop_at then
+      if List.physically_equal l stop_at then
         List.rev acc
       else
         match l with

src/stdune/list.ml

@@ -115,3 +115,5 @@ let rec nth t i =
   | [], _ -> None
   | x :: _, 0 -> Some x
   | _ :: xs, i -> nth xs (i - 1)
+
+let physically_equal = Pervasives.(==)

src/stdune/list.mli

@@ -43,3 +43,5 @@ val assoc : ('a * 'b) t -> 'a -> 'b option
 val singleton : 'a -> 'a t
 
 val nth : 'a t -> int -> 'a option
+
+val physically_equal : 'a t -> 'a t -> bool