The `Game` class now exists

This is the class that should handle basic message passing.
I'm aware it's probably going to become something of a God
Object - but there's ways to make that not happen, I'd
presume. I need something to run the main game logic loop,
regardless, and to pass around messages.

Planning to use std.signals or sth for them messages.
Game in its own module now to prevent reinit

It's supposed to be a singleton.