* gc.h (rb_objspace_reachable_objects_from_root): added.
  This API provides information which objects are root objects.
  `category' shows what kind of root objects.
* gc.c (gc_mark_roots): separate from gc_marks_body().



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43294 b2dd03c8-39d4-4d8f-98ff-823fe69b080e