port over sgen's idea of LOSSections as a mid-sized arena, so that we now have:

SmallArena original non-large allocator, free bitmaps, segregated-fit allocator.
  handles objects where size <= 3584 bytes
LargeArena (new code, size-specific free lists.)
  handles object  where 3584 < size <= ~1 meg
HugeArena (original large allocator, 1 mmap per object.
  handles objects where size > ~1meg