Split crate into lib and bin chunks

I'm going to be doing some silly WASM and ES Module stuff here shortly,
so I need a cdylib. This means I'll need the crate to actually have a
library in it.

src/lib.rs

@@ -0,0 +1,3 @@
+pub mod primitives;
+pub mod renderer;
+pub mod scene;

src/main.rs

@@ -1,11 +1,7 @@
-mod primitives;
-mod renderer;
-mod scene;
+use rustpt::primitives::{Vec2i, Vec3};
+use rustpt::scene::{Camera, Scene};
 
-use crate::primitives::{Vec2i, Vec3};
-use crate::scene::{Camera, Scene};
-
-use crate::renderer::{RenderProperties, Tile};
+use rustpt::renderer::{RenderProperties, Tile};
 
 use rand::SeedableRng;
 use rand::rngs::SmallRng;