stdlib: implement List.is_empty using pattern-matching

As of ocaml#10681, the native compiler is able to simplify this
pattern-matching to use bitwise operations rather than an explicit
branch (as was previously the case) or an integer comparison (as is the
case for the implementation using `( = )`).

stdlib/list.ml

@@ -549,7 +549,9 @@ let rec compare_length_with l n =
       compare_length_with l (n-1)
 ;;
 
-let is_empty l = (l = [])
+let is_empty = function
+  | [] -> true
+  | _ :: _ -> false
 
 (** {1 Comparison} *)