Changes

Optimize Navigation Listener

Replace WeakHashMap with an ArrayList and manually manage object
lifecycle. Gives superior iteration performance at a slight cost
to removal performance and also ensures entities are removed immediately
upon losing their validity within the world.

Additionally, change listener registration to be done upon world add
instead of immediate up creation. This provides benefit of only
registering and ticking real Navigation objects, and not invalid
entities (cancelled entity spawns for example).