Identity "Generation" is unused

[petergiuntoli]

Entities have a "generation" associated with them but it is always set to 1 since nothing passes a generation in to the constructor.

RelEcs/src/Identity.cs

Lines 27 to 32 in a9051ae

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public Identity(int id, ushort generation = 1)
	{
	Id = id;
	Generation = generation;
	}

Inside Despawn() we queue up the "unused" id without incrementing the generation

RelEcs/src/Archetypes.cs

Line 74 in a9051ae

UnusedIds.Enqueue(identity);

and then inside Spawn() we pop that id without incrementing the generation

RelEcs/src/Archetypes.cs

Line 37 in a9051ae

var identity = UnusedIds.Count > 0 ? UnusedIds.Dequeue() : new Identity(++EntityCount);

Additionally, functions like IsAlive() do not check the generation at all

RelEcs/src/Archetypes.cs

Lines 282 to 285 in a9051ae

	internal bool IsAlive(Identity identity)
	{
	return Meta[identity.Id].Identity != Identity.None;
	}

It's easy to reproduce this issue since the despawned ids are used immediately

var entity = world.Spawn().Id();
world.Despawn(entity);
var anotherEntity = world.Spawn().Id();
Assert.IsFalse(entity == anotherEntity); //entity == anotherEntity will return true
Assert.IsTrue(world.IsAlive(anotherEntity));
Assert.IsFalse(world.IsAlive(entity)); // This will of course also be true unexpectedly

The same issue exists in HypEcs Byteron/HypEcs#4