variable.c: refactor accesses to the generic_fields_tbl

All accesses to `generic_fields_tbl_` are now encapsulated inside:

  - `rb_obj_fields`
  - `rb_obj_set_fields`
  - `rb_obj_replace_fields`

Author: byroot

internal/variable.h

@@ -46,7 +46,6 @@ void rb_gvar_namespace_ready(const char *name);
  */
 VALUE rb_mod_set_temporary_name(VALUE, VALUE);
 
-int rb_gen_fields_tbl_get(VALUE obj, ID id, VALUE *fields_obj);
 void rb_obj_copy_ivs_to_hash_table(VALUE obj, st_table *table);
 void rb_obj_init_too_complex(VALUE obj, st_table *table);
 void rb_evict_ivars_to_hash(VALUE obj);

ractor.c

@@ -1679,8 +1679,7 @@ obj_traverse_replace_i(VALUE obj, struct obj_traverse_replace_data *data)
 } while (0)
 
     if (UNLIKELY(rb_obj_exivar_p(obj))) {
-        VALUE fields_obj;
-        rb_ivar_generic_fields_tbl_lookup(obj, &fields_obj);
+        VALUE fields_obj = rb_obj_fields_no_ractor_check(obj);
 
         if (UNLIKELY(rb_shape_obj_too_complex_p(obj))) {
             struct obj_traverse_replace_callback_data d = {