Function root_to_leaves 

pub fn root_to_leaves<F, Fut>(
    root_directory_digest: B3Digest,
    get_directory: F,
) -> impl Stream<Item = Result<Directory, Error>> + use<F, Fut>
where
    F: Fn(B3Digest) -> Fut + Sync + Send + 'static,
    Fut: Future<Output = Result<Option<Directory>, Error>> + Send,

Traverses a Directory from the root to the children.

This is mostly BFS, but directories are only returned once.