gc.c: rename heap.free_num to heap.swept_num

* gc.c: rename heap.free_num as heap.swept_num to clarify meaning and
  avoid confusion with objspace_free_num().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43140 b2dd03c8-39d4-4d8f-98ff-823fe69b080e