Add node cnt

This PR adds the number of nodes to the node's metadata.
Storing the number of nodes of a subtree will effectively speed up some operations.
You can't propagate the balance level upwards (to the root node) in a trivial way. Having the real (maximum) height would be the better choice. It's more easily propagatable and, together with the node-count, you can derive whether _all_ the subtrees within a subtree are balanced or not.