Labels should support arbitrary tree hierarchies

[Sulochan Acharya]

We should use materialized paths to make such lookups efficient, at minor cost for updates.

Variables overrides should chain with respect to such hierarchies.

Such support allows for mapping network device topologies, so long as they are strict trees. However, a device will continue to have support for multiple labels, which should give us most of the benefits of arbitrary DAGs without having to compute potentially expensive transitive closures.