I wonder about the following solution. At least it traverses the
hashtable exactly once (and it does not ignore the result of the
fold).


let to_array9 t =
  let Some (a,_) =
    Hashtbl.fold (fun k v seed ->
      match seed with
	Some (a,i) -> a.(i) <- (k,v); Some (a,i+1)
      | None -> let a =  Array.make (Hashtbl.length t) (k,v) in
                Some (a,1))
      t None
  in a
;;