Improve Wagner build performace by adding virtual node data to edges labels

[recursion-ninja]

The Wagner build is currently very slow because all information from the previous tree is discarded when build the new candidate trees. We can improve this by adding a virtual node datum on each edge and compare adding a new node on that edge by using the postorder logic between the virtual node datum and the new leaf node datum to get the cost of adding that leaf node on that edge without rebuilding the whole tree. This will improve performance by an order of magnitude.

[recursion-ninja]

The Wagner build implementation has been improved to reduce the asymptotic runtime from O(n^3) to O(n^2). See 59c4f2d52d88f23c0338db2a7acafb5a2f474c37.