A tagging interface for an object representing the entity in the game that
is going to moving along the path. This allows us to pass around entity/state
information to determine whether a particular tile is blocked, or how much
cost to apply on a particular tile.
For instance, a Mover might represent a tank or plane on a game map. Passing round
this entity allows us to determine whether rough ground on a map should effect
the unit's cost for moving through the tile.