pub struct Intersection<'a> {
    pub distance: f64,
    pub object: &'a Sphere,
}
impl<'a> Intersection<'a> {
    pub fn new<'b>(distance: f64, object: &'b Sphere) -> Intersection<'b> {
        // Elided
    }
}
impl Scene {
    pub fn trace(&self, ray: &Ray) -> Option<Intersection> {
        self.spheres
            .iter()
            .filter_map(|s| s.intersect(ray).map(|d| Intersection::new(d, s)))
            .min_by(|i1, i2| i1.distance.partial_cmp(&i2.distance).unwrap())
    }
}