* gc.c: move increment from heap to heap_pages.
  Share `increment' information with heaps.
* gc.c: change ratio of heap_pages_free_min_page
  to 0.80.
  This change means slow down page freeing speed.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43397 b2dd03c8-39d4-4d8f-98ff-823fe69b080e