Use heuristic for quick accept without cycle check in `add_edge`

[bluss]

The type invariant that the current DAG is acyclic can be used to speed up incremental checks. A full algorithm is still O(|V| + |E|) though.

Some easy heuristics are:

• is either a or b without prior edges?
• is there already an edge from a to b with the same directionality?

[bluss]

I saw add_child which says

    /// This is faster than using `add_node` and `add_edge`. This is because we don't have to check
    /// if the graph would cycle when adding an edge to the new node, as we know it it will be the
    /// only edge connected to that node.

but that a node has no edges is something we can very easily check in add_edge too.

[bluss]

🎉