/// Returns the hash of the best block and, optionally, its height.
///
/// When polling a block source, [`Poll`] implementations may pass the height to [`get_header`]
/// to allow for a more efficient lookup.
///
/// Returns the hash of the best block and, optionally, its height.
///
/// When polling a block source, [`Poll`] implementations may pass the height to [`get_header`]
/// to allow for a more efficient lookup.
///