Move asteroid bits to another module

Now to fix the visibility issues and make the program compile again...

src/asteroids.rs

@@ -0,0 +1,51 @@
+/// This is the module containing all the rock-related things
+/// not... not the whole game.
+use bevy::prelude::*;
+
+use crate::{GameAssets, Position, Rotation, Velocity};
+
+#[derive(Component, Deref, DerefMut)]
+struct Asteroid(AsteroidSize);
+
+enum AsteroidSize {
+    SMALL,
+    MEDIUM,
+    LARGE,
+}
+
+#[derive(Resource)]
+struct AsteroidSpawner {
+    timer: Timer,
+    // TODO: Configurables?
+    // - interval
+    // - density
+    // - size distribution
+}
+
+/// Update the asteroid spawn timer and spawn any asteroids
+/// that are due this frame.
+fn tick_asteroid_manager(
+    mut commands: Commands,
+    mut spawner: ResMut<AsteroidSpawner>,
+    game_assets: Res<GameAssets>,
+    time: Res<Time>,
+) {
+    spawner.timer.tick(time.delta());
+    if spawner.timer.just_finished() {
+        commands.spawn((
+            Asteroid(AsteroidSize::SMALL),
+            Position(Vec2::new(40.0, 40.0)),
+            Velocity(Vec2::new(10.0, 0.0)),
+            Rotation(0.0),
+            Mesh2d(game_assets.asteroid_small().0),
+            MeshMaterial2d(game_assets.asteroid_small().1),
+        ));
+    }
+}
+
+/// Utility function to spawn a single asteroid of a given type
+/// TODO: convert to an event listener monitoring for "spawn asteroid" events
+/// from the `fn tick_asteroid_manager(...)` system.
+fn spawn_asteroid(mut commands: Commands) {
+    todo!();
+}