Ocaml native int63 extraction

Hello everybody,

I have a file Sample.v with the content

Require Import String.
Require Import Int63.Sint63.
Require Import PArray.
Require Import Ascii String Coq.Strings.Byte.

Module Sample.

Definition bytes : Set := string.

Definition len_bytes (s:bytes) := 1+ length s.

End Sample.

and a file with the content

Require Export Sample.
Require Extraction.
Require ExtrOcamlBasic.
Require ExtrOCamlInt63.
Require ExtrOcamlChar.
Require ExtrOcamlNativeString.

Extraction Language OCaml.

Extraction Library Sample.

When I extract the Ocaml code I get

open Datatypes
open Nat
open String

module Sample =
type bytes = string

(** val to_bytes : string → nat **)

let len_bytes s =
add (S O) (length s)

And now my question comes. Where I can find the compiled code/library/interfaces
for the modules Nat, Datatypes and String?
Of course I can extract them and compile myself, but this triggers generation of
other modules. This is actually good enough, but maybe there is some simpler
way to compile my extracted sample (precompiled libraries or something). I have
already spent a good deal of time looking for a solution, but I did not find one.

All the best,