De-boxed many uses of Hit and Material.

Use of generic parameter directly where possible in structures instead
of Box'd values.
Added Material implementations for Box<Material> and Arc<Material> to
aid in the automatic conversion when necessary to use a Sized value for
Material.
Implement From trait for [f32;3] to Vec3 to make some APIs Into<Vec3>
which is a bit nicer to use.

rtiow/src/flip_normals.rs

@@ -3,17 +3,26 @@ use hitable::Hit;
 use hitable::HitRecord;
 use ray::Ray;
 
-pub struct FlipNormals {
-    hitable: Box<Hit>,
+pub struct FlipNormals<H>
+where
+    H: Hit,
+{
+    hitable: H,
 }
 
-impl FlipNormals {
-    pub fn new(hitable: Box<Hit>) -> FlipNormals {
+impl<H> FlipNormals<H>
+where
+    H: Hit,
+{
+    pub fn new(hitable: H) -> FlipNormals<H> {
         FlipNormals { hitable }
     }
 }
 
-impl Hit for FlipNormals {
+impl<H> Hit for FlipNormals<H>
+where
+    H: Hit,
+{
     fn hit(&self, r: Ray, t_min: f32, t_max: f32) -> Option<HitRecord> {
         if let Some(rec) = self.hitable.hit(r, t_min, t_max) {
             return Some(HitRecord {