Implementing borrow for fixed size types

Right now most types implement AsRef like Vector3 implements AsRef<[T;3]>.

There's a case where being able to accept both a &[f32;3] or a Vec3 would be useful. For example to create universal apis that can work with any library like:

```rs
fn clear_color>(c: &C)
```

can be called as:

```rs
clear_color( &[1., 1., 1., 1.]);
clear_color(&na::Vector4(1., 1., 1., 1.));
clear_color(&color::Rgba(1., 1., 1., 1.));
```

while AsRef only would support the last 2 cases, it's impossible to take &[f32;4] as AsRef without having some really verbose trait implementation while Borrow has a blanket implementation for any &T.

This would allow to create apis that could take any type with the correct memory structure without having dependencies on any specific library.

Not sure though if this is a correct usage of Borrow or there's a better way to accomplish the same.

I can create a PR request implementing Borrow for any type where it makes sense if this is something that seems useful