Complete rewrite of the NPC and version-specific code

[Techcable]

I feel like the NPC code is the most fragile part of the codebase.
It's been getting better, but I think I need to move all of the pet-related logic out of the NPC logic for cleanliness.
Interaction with the new NPC code will simply involve listening for events and .
The new interface must cleanly support multiple NPCs and human NPCs.
It must be lightweight, of high quality, and well-encapsulated so that someday I can pull it out and maintain it as a separate library.
Instead of using bytecode generation or magic template libraries, I intend to use readable python code to generate the version-specific code.

This will be the first of many improvements on the way to v2.0.0 and will pave the way for multiple pets, human pets, and much quicker updates to new Minecraft versions.