Move object_id in object fields.

And get rid of the `obj_to_id_tbl`

It's no longer needed, the `object_id` is now stored inline
in the object alongside instance variables.

We still need the inverse table in case `_id2ref` is invoked, but
we lazily build it by walking the heap if that happens.

The `object_id` concern is also no longer a GC implementation
concern, but a generic implementation.

Co-Authored-By: Matt Valentine-House <matt@eightbitraptor.com>

Author: byroot

ext/objspace/objspace_dump.c

@@ -805,29 +805,29 @@ shape_i(rb_shape_t *shape, void *data)
     dump_append(dc, ", \"depth\":");
     dump_append_sizet(dc, rb_shape_depth(shape));
 
-    dump_append(dc, ", \"shape_type\":");
     switch((enum shape_type)shape->type) {
       case SHAPE_ROOT:
-        dump_append(dc, "\"ROOT\"");
+        dump_append(dc, ", \"shape_type\":\"ROOT\"");
         break;
       case SHAPE_IVAR:
-        dump_append(dc, "\"IVAR\"");
+        dump_append(dc, ", \"shape_type\":\"IVAR\"");
 
         dump_append(dc, ",\"edge_name\":");
         dump_append_id(dc, shape->edge_name);
 
         break;
       case SHAPE_FROZEN:
-        dump_append(dc, "\"FROZEN\"");
+        dump_append(dc, ", \"shape_type\":\"FROZEN\"");
         break;
       case SHAPE_T_OBJECT:
-        dump_append(dc, "\"T_OBJECT\"");
+        dump_append(dc, ", \"shape_type\":\"T_OBJECT\"");
         break;
       case SHAPE_OBJ_TOO_COMPLEX:
-        dump_append(dc, "\"OBJ_TOO_COMPLEX\"");
+        dump_append(dc, ", \"shape_type\":\"OBJ_TOO_COMPLEX\"");
+        break;
+      case SHAPE_OBJ_ID:
+        dump_append(dc, ", \"shape_type\":\"OBJ_ID\"");
         break;
-      default:
-        rb_bug("[objspace] unexpected shape type");
     }
 
     dump_append(dc, ", \"edges\":");