Switching to a Military Themed Ranking System

Previously we had Node with methods LinkTo and UnlinkTo, which weren't
really descriptive.

Now we have Rank, with methods AddSuperior and RemoveSuperior. Superiors
must always outrank subordinates.

Keeping with the military theme, EnsureHigherPriorityThan has been
renamed to Outrank, and accepts a Rank instead of the Ranks priority.