[patch] arena rework - unify arenaroots

folks,

Heres a resend (may have tiny diffs) of previously sent patch.
It puts all (body) arenas onto a single linked-list.
It modifies intrpvar.h, so needs a regen

• [patch] arena rework - unify arenaroots by Jim Cromie
• Re: [patch] arena rework - unify arenaroots by Nicholas Clark