* gc.c: fix heaps_header and heaps_slot to reduce memory consumption.
  (1) move heaps_header::start and limit to  heaps_slot.
  (2) remove heaps_header::end which can be calculated by start+limit.
* gc.c: catch up above change.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42020 b2dd03c8-39d4-4d8f-98ff-823fe69b080e