rb_thread_t::local_storage_recursive_hash_for_trace to store
  recursive hash to avoid creating new recursive (nested) hashes
  for each trace events.
  [Bug #10511]
* vm_trace.c (rb_threadptr_exec_event_hooks_orig): use it.
* cont.c: catch up this fix.
* vm.c (rb_thread_mark): ditto.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e