remove_all can exhaust file descriptor limit in extreme cases

[cyphar]

When running the 128 thread tests of Root::remove_all it's possible to exhaust the default 1024 open files ulimit. For tests we can raise the ulimit, but maybe keeping an open handle for every element of the path up to the root for the path we are currently operating on is not ideal. Unfortunately it's not clear if switching to storing the paths (and then re-doing the lookup) is better overall (it would definitely make removals more expensive).

The Go stdlib RemoveAll and Rust remove_dir_all implementations have the same issue, for what it's worth.