Several objectives:
1. Reduce humongous allocation failures by segregating regular regions from humongous regions
2. Do not retire regions just because an allocation failed within the region if the memory remaining within the region is large enough to represent a LAB
3. Track range of empty regions in addition to range of available regions in order to expedite humongous allocations
4. Treat collector reserves as available for Mutator allocations after evacuation completes
5. Improve encapsulation so as to enable an OldCollector reserve for future integration of generational Shenandoah