This patch fixes the latter part of #3710. We need to change
`struct REnv` to fix the former part of the issue.

The behavior of `OP_RAISE` has been changed.

Fix a typo in gc.c.

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.