remove %caml_string_set16u, use the bytes equivalent

[hhugo]

this is in preparation for ocsigen/js_of_ocaml#923

[hhugo]

Note that the patch doesn't work with OCaml < 4,07.

[dinosaure]

Thanks, let me think about a compatibility way 👍

[hhugo]

With a light https://github.com/ocsigen/js_of_ocaml/blob/master/compiler/ppx/ppx_optcomp_light.ml
version of ppx_optcomp

Here is how jsoo does it.

external unsafe_set_uint16 : bytes -> int -> int -> unit = "%caml_string_set16u"
  [@@noalloc] [@@if ocaml_version < (4, 7, 0)]

external unsafe_set_uint16 : bytes -> int -> int -> unit = "%caml_bytes_set16u"
  [@@noalloc] [@@if ocaml_version >= (4, 7, 0)]

[dinosaure]

Ok, I will try to propose a PR with your commit, thanks!

[dinosaure]

Close by #37.