Improve iteration complexity

Provide a `read_dir_nth()` syscall that starts iteration not at the first file. Same for `read_dir_files_nth()`. This can reduce the time taken by paging mechanisms like in our [secrets-app](https://github.com/Nitrokey/trussed-secrets-app/blob/main/src/authenticator.rs#L428).

This could be further optimized by https://github.com/trussed-dev/littlefs2/issues/32