Arena can be modified during the marking phase.

Because the root context cannot be marked from fibers.

Mostly for debugging purpose.

Otherwise the function may terminate and cause memory leaks.

This means #3701 is now OK to merge.

On my machine, gdb reverse-next does not work with memcpy and memset.
Add naive implementation of those functions to override.

And arena-overflow error as well.  They should not be caught by
`rescue` by default.

This reverts commit eb5a606f and
079f310f.
The rescue stack works differently from ensure stack, so the change
caused #3686 and #3688. It might take long to solve the problems,
so that I would revert the changes for now. Fix #3688

This reverts commit 31e30686.