* gc.c: change terminlogy "..._num" to "..._slots" about slot opetaion.
  * final_num -> final_slots
  * objspace_live_num() -> objspace_live_slots()
  * objspace_limit_num() -> objspace_limit_slots()
  * objspace_free_num() -> objspace_free_slots()



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43836 b2dd03c8-39d4-4d8f-98ff-823fe69b080e