Function directory_walker 

fn directory_walker(
    path: &Path,
) -> Filter<FilterMap<IntoIter, impl FnMut(Result<DirEntry, Error>) -> Option<DirEntry>>, impl FnMut(&DirEntry) -> bool>

A helper function to set up a directory walker within a given directory.

This ignores symlinks. This also guarantees that the entries will be walked over in an order, sorted by file name.