109.13.00

- Added `Command.Spec.flags_of_args_exn`, for compatibility with
  OCaml's standard library.

  This function converts a `Core.Std.Arg.t` into a `Command.Spec.t`.
- Made various modules `Identifiable`: `Char`, `String`, and the
  various `Int` modules.

  In particular, `Int` being identifiable is useful, because one can
  now write:

  ```ocaml
  module My_numeric_identifier : Identifiable ` Int
  ```

  You might think that we could now delete `String_id`, and just
  write:

  ```ocaml
  module My_string_identifier : Identifiable ` String
  ```

  But this is not quite equivalent to using `String_id`, because
  `String_id.of_string` enforces that its argument is nonempty.

- Removed module `Space_safe_tuple`, which became unnecessary in OCaml
  4.00.0.

  OCaml 4.00.0 included Fabrice's patch to fix the space leak that
  `Space_safe_tuple` was circumventing (PR#5288, commit SVN 11085).
- Added `Exn.to_string_mach`, for single-line output.
- Added `Linux_ext.bind_to_interface`, to improve security of UDP
  applications.

  ```ocaml
  val bind_to_interface : (File_descr.t -> string -> unit) Or_error.t
  ```

  This uses the linux-specifc socket option `BINDTODEVICE` to prevent
  packets being received from any interface other than one named.
- Fixed `Unix.mkdir_p` on Mac OS X.