* gc.c: introduce macros to remove magic number.
  GC_HEAP_FREE_SLOTS_MIN_RATIO = 0.3: guarantee minimum empty slots
                                      ratio after sweep.
  GC_HEAP_FREE_SLOTS_MAX_RATIO = 0.8: allow to free pages 0.2 (= 1-0.8)
                                      of current existing slots.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45948 b2dd03c8-39d4-4d8f-98ff-823fe69b080e