Use Util.Persistent to reimplemented promoted files store

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

src/build_system.ml

@@ -10,6 +10,11 @@ let alias_dir = Path.(relative build_dir) ".aliases"
 let misc_dir = Path.(relative build_dir) ".misc"
 
 module Promoted_to_delete = struct
+  module P = Utils.Persistent(struct
+      type t = Path.t list
+      let name = "PROMOTED-TO-DELETE"
+      let version = 1
+    end)
   let db = ref []
 
   let fn = Path.relative Path.build_dir ".to-delete-in-source-tree"
@@ -17,19 +22,15 @@ module Promoted_to_delete = struct
   let add p = db := p :: !db
 
   let load () =
-    if Path.exists fn then
-      Io.Sexp.load fn ~mode:Many
-      |> List.map ~f:(Sexp.Of_sexp.parse Path.t Univ_map.empty)
-    else
-      []
+    Option.value ~default:[] (P.load fn)
 
   let dump () =
-    let db = Path.Set.union (Path.Set.of_list !db) (Path.Set.of_list (load ())) in
     if Path.build_dir_exists () then
-      Io.write_file fn
-        (String.concat ~sep:""
-           (List.map (Path.Set.to_list db) ~f:(fun p ->
-              Sexp.to_string ~syntax:Dune (Path.sexp_of_t p) ^ "\n")))
+      load ()
+      |> Path.Set.of_list
+      |> Path.Set.union (Path.Set.of_list !db)
+      |> Path.Set.to_list
+      |> P.dump fn
 end
 
 let files_in_source_tree_to_delete () =