Add Direction2d::from_xy and Direction3d::from_xyz

[Jondolf]

Objective

Make direction construction a bit more ergonomic.

Solution

Add Direction2d::from_xy and Direction3d::from_xyz, similar to Transform::from_xyz:

let dir2 = Direction2d::from_xy(0.5, 0.5).unwrap();
let dir3 = Direction3d::from_xyz(0.5, 0.5, 0.5).unwrap();

This can be a bit cleaner than using new:

let dir2 = Direction2d::new(Vec2::new(0.5, 0.5)).unwrap();
let dir3 = Direction3d::new(Vec3::new(0.5, 0.5, 0.5)).unwrap();

[alice-i-cecile]

@NiseVoid can I get your review here?

[NiseVoid]

Looks good to me, having to wrap things in VecX::new(...) can get quite ugly if your code happens to not work with them already.