remove oo_id, use Obj.field, "%field1" is dedicated for tuple

Author: bobzhang

jscomp/runtime/bs_stdlib_mini.mli

@@ -61,6 +61,7 @@ module Obj : sig
   (* The compiler ensures (|0) operation *)
   external set_tag : t -> int -> unit = "tag" [@@bs.set]  
   external repr : 'a -> t = "%identity"
+  external obj : t -> 'a = "%identity"
   external magic : 'a -> 'b = "%identity"  
   external size : t -> int = "#obj_length"
 end 

jscomp/runtime/caml_hash.ml

@@ -79,8 +79,6 @@ let unsafe_pop (q : 'a t) =
 external ( +~ ) : nativeint -> nativeint -> nativeint =
    "caml_int32_add"
 
-(*ATTENTION: refer {!Oo.id} *)
-external oo_id : Obj.t -> int  = "%field1"
 
 open Caml_hash_primitive
 
@@ -134,7 +132,8 @@ let caml_hash (count : int) _limit (seed : nativeint)
           let obj_tag = Obj.tag obj in
           let tag = (size lsl 10) lor obj_tag in 
           if tag = 248 (* Obj.object_tag*) then 
-            hash.contents <- caml_hash_mix_int hash.contents (Caml_nativeint_extern.of_int (oo_id  obj))
+            hash.contents <- caml_hash_mix_int hash.contents 
+              (Caml_nativeint_extern.of_int (Obj.obj (Obj.field obj 1) : int))
           else 
             begin 
               hash.contents <- caml_hash_mix_int hash.contents (Caml_nativeint_extern.of_int tag) ;