distance_squared should work with integers

It looks like the ``distance_squared`` function only works with floats. It makes sense that the ``distance`` function does not work with integers due to the square rooting, but ``distances_squared`` does not have this limitation. Any input with only integers gives an integer output.

Throughout my code I have to constantly cast back and forth between ``Point`` and ``Point`` like:

```
let coords_as_f32 = Point3::new(coords.x as f32, coords.y as f32, coords.z as f32);
let chunk_pos_as_f32 = Point3::new(chunk_pos.x as f32, chunk_pos.y as f32, chunk_pos.z as f32);
if nalgebra::distance_squared(&coords_as_f32, &chunk_pos_as_f32) < RENDER_DISTANCE.pow(2) as f32 {
// ...
```

Which could be simplified to the following, if this arbitrary restriction was removed:

```
if nalgebra::distance_squared(&coords, &chunk_pos) < RENDER_DISTANCE.pow(2) {
// ...
```

Let me know if I am missing something, or just overlooked an existing function somewhere.