Support for very deep file hierarchies

Currently we don't properly support certain pathological cases like described in uutils/coreutils#7376 since we use similar standard library primitives.

This is currently blocked at wrappers for `*at` POSIX functions (e.g. [`openat`][1]) not being in the standard library: rust-lang/rust#120426. Because it can take a very long time until this is implemented and stabilized, we could implement our own wrappers in the meantime.

[1]: https://linux.die.net/man/2/openat