Add debugging mode that clears memory to Immix

This overwrites memory with all 1's in immix when a line is not used.
This makes it easier to debug because objects will be seen as invalid
later if they haven't been properly marked.
Sweep after unremembering objects

This makes sure it works with the Immix GC debugging flag because
otherwise the objects are already invalid in the unremember set.
Update current finalizer if a GC is triggered