Because of callbacks, calls into Zig can be nested. Previously, the call_arena
was reset after _every_ call. When calls are nested, this doesn't work - the
nested call resets the arena, which the caller might still need. A `call_depth`
integer was added to the Executor. Each call starts by incrementing the
call_depth and, on deinit, decrements the call_depth. The call_arena is only
reset when the call_depth == 0. This could result in lower memory use.

Also promoted the call_arena and scope_arena to the Env. Practically speaking,
nothing has changed, since they're still reset under the exact same conditions.
However, when an executor ends and a new one is started, it can now reuse the
retained_capacity of the previous arenas. This should result in fewer
allocations.

The call_arena is still re-added, and the call_depth is still used, but
the call_arena and scope_arenas are no longer part of the Env - they remain on
the Executor.

This is to accommodate upcoming changes where multiple executors will exist at
once. While the shared allocators _might_ have been safe in some cases, the
performance gains don't justify the risk of having 2 executors interacting in a
way where sharing the allocators would cause issues.