Function astar_search_digraph 

pub fn astar_search_digraph<N, E, Ix, H>(
    graph: &DiGraph<N, E, Ix>,
    source: &N,
    target: &N,
    heuristic: H,
) -> Result<AStarResult<N, E>>
where
    N: Node + Debug + Clone + Hash + Eq,
    E: EdgeWeight + Clone + Add<Output = E> + Zero + PartialOrd + Copy,
    Ix: IndexType,
    H: Fn(&N) -> E,

A* search for directed graphs