Allow passing OCaml values directly to stubs

[aantron]

I want to call

void get_sockaddr(value mladr,
                  union sock_addr_union * adr /*out*/,
                  socklen_param_type * adr_len /*out*/);

...but I found no way to pass the value argument in current ctypes.

So, this PR adds:

val ocaml_any_value : 'a typ
(** Value representing any OCaml value, to be accessed from C directly. This
    corresponds to the C type [value] from [caml/mlvalues.h]. *)

...which is used like this:

(* type t defined elsewhere *)
let unix_sockaddr : Unix.sockaddr typ = ocaml_any_value

let get =
  foreign "get_sockaddr"
    (unix_sockaddr @-> ptr t @-> ptr int @-> returning void)

The ML code generated for get is:

external luv_stub_63_get_sockaddr : 'a -> _ CI.fatptr -> _ CI.fatptr -> unit
  = "luv_stub_63_get_sockaddr"

(* ... *)

| Function
    (CI.OCaml_value,
     Function (CI.Pointer _, Function (CI.Pointer _, Returns CI.Void))),
  "get_sockaddr" ->
  (fun x13 x14 x15 ->
    luv_stub_63_get_sockaddr x13 (CI.cptr x14) (CI.cptr x15))

...and the C stub is:

value luv_stub_63_get_sockaddr(value x270, value x269, value x268)
{
   union sock_addr_union* x272 = CTYPES_ADDR_OF_FATPTR(x269);
   int* x273 = CTYPES_ADDR_OF_FATPTR(x268);
   get_sockaddr(x270, x272, x273);
   return Val_unit;
}

These look right to me, and seem to work. I'm not sure about some of the other code this PR touches, like in cstubs_analysis.ml.

[yallop]

I understand the aim of this PR, but passing OCaml values to C can be quite subtle, and it's rarely needed, so I think the cost/benefit balance is against adding this feature.

[aantron]

This tends to be useful also for calling the runtime's Bigarray helpers. I find it very helpful for developing https://github.com/aantron/luv. I guess without this PR, I would have to use regular external declarations, which I'm trying to minimize.

[bobot]

It would simplify a lot #643, where one wants to communicate between two stubs, one being [zarith.h] (https://github.com/ocaml/Zarith/blob/master/zarith.h). I believe it is useful for any stubs that provide conversion function from the C type to/from the OCaml type.

I tried to add the feature on the current master just as Obj.t typ for simplicity (views should allow to generalize it), with a test at https://github.com/bobot/ocaml-ctypes/tree/obj_t_typ. It works with stubs but not with foreign even if there are fixes compared to this MR on the handling of those values as argument and result. (EDIT: the error is fixed, the example works)