Ammo spawning

The Ammo prefab created so far presents us with a technical problem that, if not taken seriously, has the potential to cause some serious performance penalties for our game. Specifically, when the spaceship weapon is fired, we'll need to generate ammo that launches into the scene and destroys the enemies on collision. This is fine in general, but the problem is that the player could potentially press the fire button many times in quick succession and could even hold down the fire button for long periods of time, and thereby spawn potentially hundreds of ammo prefabs. We could, of course, use the Instantiate function seen already to generate these prefabs dynamically, but this is problematic because instantiate is computationally expensive. When used to generate many items in succession, it will typically cause a nightmarish slowdown that'll reduce the FPS to unacceptable levels. We need to avoid this!

The solution is known as Pooling, Object Pooling, or Object Caching. In essence...