Reviewed by Maciej Stachowiak.

        Switched heap to use the Bitmap class and removed CollectorBitmap
        https://bugs.webkit.org/show_bug.cgi?id=53391
        
        SunSpider says 1.005x as fast. Seems like a fluke.

        * runtime/MarkedSpace.cpp:
        (JSC::MarkedSpace::allocate): Updated for rename and returning a value
        rather than taking a value by reference.

        * runtime/MarkedSpace.h: Code reuse is good.

        * wtf/Bitmap.h:
        (WTF::::testAndSet): Added, since this is the one thing Bitmap was missing
        which CollectorBitmap had. (Renamed from the less conventional "getset".)

        (WTF::::nextPossiblyUnset): Renamed and changed to return a value for
        clarity. It's all the same with inlining.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@77080 268f45cc-cd09-0410-ab3c-d52691b4dbfc