Closes#16
Bevy provides Deref and DerefMut derives, so that's nice. I'm not sure
it makes any sense to keep the field private since those derefs expose
the Vec2 anyway. I added an `impl Default` anyway, though.
I'm not sold on the name, but anyway.
This module will hold the systems that power the main game mechanics, as
well as a few extra items to support that (in particular, the Lifetime
component).
