Function dag::topological_sort

pub fn topological_sort<InIndex, OutIndex, F, OrderIndicator, FOrd, Set>(
    index_set: Set,
    predecessor_edges: F,
    order_indicator: FOrd
) -> Vec<OutIndex>
where
    InIndex: Copy + Hash + Eq + PartialEq + Ord + TryInto<OutIndex>,
    <InIndex as TryInto<OutIndex>>::Error: Debug,
    F: Fn(InIndex) -> Vec<InIndex>,
    OrderIndicator: Ord,
    FOrd: Fn(InIndex) -> OrderIndicator,
    Set: SetLike<InIndex> + Default + Clone + IntoIterator<Item = InIndex>,

Topologically sort index_set and return a sorted Vec. For the nodes without order-before relationship, the ones with smaller order_indicator output will be ordered first.