* gc.c (MARKED_IN_BITMAP, MARK_IN_BITMAP, CLEAR_IN_BITMAP): bring
	  bitmap macros in one place, and introduce BITMAP_BIT.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41126 b2dd03c8-39d4-4d8f-98ff-823fe69b080e