gc: Create a validation mode

To narrow down issues with the gc, it might be a good idea to do some extra (optional) validation before doing a sweep step, potentially even before the CallDestroyObjects step.

We can for every chunk copy its blackBitmap into an external data structure (in the simplest case, a hashmap from chunk address to bitmap), and clear it afterwards. The, run another non-incremental gc pass (only the marking part). Afterwards, compare the actual black bitset with their stored copies.

If an object is unmarked in actual bitmap, but marked in the copy, that's fine (incremental gc might end up marking more than necessary).

However, the other way around indicates a bug.