Entity cramming XP

Overview:
Entity cramming XP makes it so mobs that are killed by contact by Entity Cramming will drop their XP orbs as if killed by a player.

This allows creation of auto-kill grinders that generate XP while players are AFK, discouraging build up of entities in mob farms.

Note: XP orbs are still subject to being destroyed by lava and fire, so mob grinder design should take this into consideration.