* gc.c: rename rb_objspace_t::marked_objects to marked_slots.
* gc.c (gc_marks_start): should be clear first.
* gc.c (gc_marks_start): remembered shady objects are also marked.
* gc.c (gc_stat_internal): add heap_marked_slots.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47474 b2dd03c8-39d4-4d8f-98ff-823fe69b080e