Implements Vec3A directly (#102)

Instead of forwarding calls to Vec3 and having Vec3A wrap Vec3,
this implements all the functions in Vec3A manually. This is required
for SPRIR-V support because rust can't have repr(simd) on a type that
isn't a collection of it's basic types.

src/f32/vec3.rs

@@ -708,7 +708,8 @@ impl From<Vec3A> for Vec3 {
 
         #[cfg(vec3a_f32)]
         {
-            v.0
+            let (x, y, z) = v.into();
+            Self { x, y, z }
         }
     }
 }