* gc.c: change RGENGC_CHECK_MODE (>= 2) logic.
  Basically, make an object graph of all of living objects before and
  after marking and check status.
  [Before marking: check WB sanity]
  If there is a non-old object `obj' pointed from old object
  (`parent') then `parent' or `obj' should be remembered.
  [After marking: check marking miss]
  Traversible objects with the object graph should be marked.
  (However, this alert about objects pointed by machine context
   can be false positive. We only display alert.)
  [Implementation memo]
  objspace_allrefs() creates an object graph.
  The object graph is represented by st_table, key is object (VALUE)
  and value is referring objects. Referring objects are stored by
  "struct reflist".
* gc.c (init_mark_stack): do not use push_mark_stack_chunk() at init.
  This pre-allocation causes failure on is_mark_stask_empty()
  without any pushing.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43744 b2dd03c8-39d4-4d8f-98ff-823fe69b080e