nonmoving: Clear filled segment bitmaps during sweep
Previously we would clear the bitmaps of all filled segments during the preparatory pause. However, this means we would do O(heap size) work during the preparatory pause resulting in poor pause times. This refactors things to rather clear filled segments' bitmaps during the (concurrent) sweep phase. This means that during the preparatory pause we now only need to clear the bitmaps of segments that were recently filled due to evacuation. The idea is that instead of adding filled segments to the filled segment list, sweep rather adds them directly back to the sweep list, where they will be picked up by the next major GC.
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