What we call an arena_ind is really the index associated with some particular
set of ehooks; the arena is just the user-visible portion of that. Making this
explicit, and reframing checks in terms of that, makes the code simpler and
cleaner, and helps us avoid passing the arena itself all throughout extent code.
This lets us put back an arena-specific assert.
This will eventually completely wrap the eset, and handle concurrency,
allocation, and deallocation. For now, we only pull out the mutex from the
eset.
