gc.c: promote long-lived NODE_CREF objects to oldgen

* include/ruby/ruby.h: add RGENGC_WB_PROTECTED_NODE_CREF setting
  In a large app, this reduces the size of
  remembered_shady_object_count by 80%. [Bug #9225] [ruby-core:58947]
* gc.c (rb_node_newnode): add FL_WB_PROTECTED flag to NODE_CREF
* class.c (rewrite_cref_stack): insert OBJ_WRITE for NODE_CREF
* iseq.c (set_relation): ditto
* iseq.c (rb_iseq_clone): ditto
* vm_eval.c (rb_yield_refine_block): ditto
* vm_insnhelper.c (vm_cref_push): ditto
* vm_insnhelper.h (COPY_CREF): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44059 b2dd03c8-39d4-4d8f-98ff-823fe69b080e