* gc.c: introduce tomb heap.
  Tomb heap is where zombie objects and ghost (freed slot) lived in.
  Separate from other heaps (now there is only eden heap) at sweeping
  helps freeing pages more efficiently.
  Before this patch, even if there is an empty page at former phase
  of sweeping, we can't free it.
  Algorithm:
    (1) Sweeping all pages in a heap and move empty pages from the
        heap to tomb_heap.
    (2) Check all exsisting pages and free a page
        if all slots of this page are empty and
           there is enough empty slots (checking by swept_num)
  To introduce this pach, there are several tuning of GC parameters.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43395 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* gc.c (heap_pages_free_unused_pages): cast to (int) for size_t
  variable `i'.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43396 b2dd03c8-39d4-4d8f-98ff-823fe69b080e