Enemy AI gets a better brain

Just a small update, following the previous fairly large one.

Now, the enemy AI now uses 8 directional movement and has a smarter brain. Enemies will now chase but give up after a while if they lose sight of the player. 

I also made some adjustments to the snappiness of the player movement - a few precise optimisations, which I hope will feel very good! Particularly on Windows. A technical musing: I wrote this in Python, but I went with Tkinter instead of the gaming libraries, which has been fun and opened up new possibilities, but also made it P R E T T Y H A R D in other ways to work with graphics that update constantly.

Enjoy!