Adding clarifying bracket.
Removing unused fields from VMArenaStruct.
Fixing CBSFindFirstInZones to return a res, because it can fail when it can’t allocate a CBS block, unlike CBSFindFirst.
Fixing related corner case in arenaAllocFindInFreeCBS when it thought it couldn’t find a block when it was in fact running out of nodes.  This was revealed by “gcbench --npass 1 --arena-size 1M --seed 945632066 --niter 1 amc”.
Fixing draft ArenaFreeCBSDelete after exercising it.