* gc.c: change terminology of heap.
  Change "slot" to "page". "Slot" is a space of RVALUE.
  1. "Heap" consists of a set of "heap_page"s (pages).
  2. Each "heap_page" has "heap_page_body".
  3. "heap_page_body" has RVALUE (a.k.a. "slot") spaces.
  4. "sorted" is a sorted array of "heap_page"s, sorted
  by address of heap_page_body (for "is_pointer_to_heap").
  See https://bugs.ruby-lang.org/attachments/4008/data-heap_structure.png.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43350 b2dd03c8-39d4-4d8f-98ff-823fe69b080e