- Replaced per-block bitmap with free list.
	- Increased number of empty blocks kept around to 2.
	- Doubled block size.
	- When scanning heap in collector, skip scanning the rest of a
	block as soon as we see as many live cells as the the number of
	used cells it had originally.

	Also the following collector changes unrelated to performance:

	- Made constants `const int' instead of `static const int'.
	- Miscellaneous code cleanup.

        * kjs/collector.cpp:

	- Added debugging mode enabled by defining DEBUG_GC which asserts
	when a destroyed ValueImp

        * kjs/internal.cpp:
        (ContextImp::mark):
        * kjs/value.cpp:
        (Value::Value):
        * kjs/value.h:
	* kjs/config.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@2845 268f45cc-cd09-0410-ab3c-d52691b4dbfc