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).