- move mark and collectOnMainThreadOnly bits into separate bitmaps
        
        This saves 4 bytes per cell, allowing shrink of cell size to 32,
        which leads to a .8% speed improvement on iBench.
        
        This is only feasible because of all the previous changes on the branch.

        * kjs/collector.cpp:
        (KJS::allocateBlock): Adjust for some renames of constants. 
        (KJS::Collector::markStackObjectsConservatively): Now that cells are 32 bytes (64 
        bytes on 64-bit) the cell alignment check can be made much more strict, and also
        obsoletes the need for a % sizeof(CollectorCell) check. Also, we can mask off the low
        bits of the pointer to have a potential block pointer to look for.
        (KJS::Collector::collectOnMainThreadOnly): Use bitmap.
        (KJS::Collector::markMainThreadOnlyObjects): Use bitmap.
        (KJS::Collector::collect): When sweeping, use bitmaps directly to find mark bits.
        * kjs/collector.h:
        (KJS::): Move needed constants and type declarations here.
        (KJS::CollectorBitmap::get): Bit twiddling to get a bitmap value.
        (KJS::CollectorBitmap::set): Bit twiddling to set a bitmap bit to true.
        (KJS::CollectorBitmap::clear): Bit twiddling to set a bitmap bit to false.
        (KJS::CollectorBitmap::clearAll): Clear whole bitmap at one go.
        (KJS::Collector::cellBlock): New operation, compute the block pointer for
        a cell by masking off low bits.
        (KJS::Collector::cellOffset): New operation, compute the cell offset for a
        cell by masking off high bits and dividing (actually a shift).
        (KJS::Collector::isCellMarked): Check mark bit in bitmap
        (KJS::Collector::markCell): Set mark bit in bitmap.
        * kjs/value.h:
        (KJS::JSCell::JSCell): No more bits.
        (KJS::JSCell::marked): Let collector handle it.
        (KJS::JSCell::mark): Let collector handle it.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@21047 268f45cc-cd09-0410-ab3c-d52691b4dbfc