feat pvec: add flat_map

src/pvec/containers_pvec.ml

@@ -352,6 +352,9 @@ let append a b =
   else
     fold_left push a b
 
+let flat_map f v : _ t =
+  fold_left (fun acc x -> append acc (f x)) empty v
+
 let rec equal_tree eq t1 t2 =
   match t1, t2 with
   | Empty, Empty -> true

src/pvec/containers_pvec.mli

@@ -82,8 +82,12 @@ val append : 'a t -> 'a t -> 'a t
 (** [append a b] adds all elements of [b] at the end of [a]. This is
   at least linear in the length of [b]. *)
 
+
 val map : ('a -> 'b) -> 'a t -> 'b t
 
+val flat_map : ('a -> 'b t) -> 'a t -> 'b t
+(** @since NEXT_RELEASE *)
+
 val choose : 'a t -> 'a option
 (** Return an element. It is unspecified which one is returned. *)