root/handle: readdir wrappers?

[cyphar]

Rust's stdlib doesn't provide a way for users to construct a ReadDir iterator (nor DirEntry) so if someone wants to implement an iterator over subtrees using libpathrs they would need to manually use rustix and adopt the logic we have for iteration in src/utils/dir.rs.

I suspect these would be Rust-only helpers since exporting them to C would be quite ugly.