angle: provide conversion trait from Vector2D

[pbor]

No description provided.

[nical]

Note that there is Vector2D::angle_from_x_axis which does this.

@kvark made a good case against implicit things in euclid in general. Let's hear what he has to say.

[kvark]

If you have this From implementation, you are getting two things:

1. Angle::from(), which is fairly straightforward, and looks good
2. Vector2D::into, which is by now way is straightforward. When looking at xx.into() passed into a function, you have no idea what it's converted into.

It's similar, in a way, to bool arguments: calling site is useless and forces you to look at the function definition.

Saying that, I think it's cleaner to just have a specific constructor aka Angle::from_vector()

[pbor]

In general I agree that using .into() can be confusing, but at the same time I find using the From trait very ergonimic instead of hunting for the correct constructor... I guess it boils down to defining a best practice where Foo::from(bar) is preferred.

Anyway, if the convention in euclid is to avoid From that is ok with me.
Let me know if you want a PR with the constructor or if I should just stick to the Vector2D method at that point (which is ok with me)

[nical]

Since there are mixed feelings and the overall sentiment is that it is fine to use the Vector2D method, I propose that we close the PR for now. We can always revisit in the future if people feel strongly about it.

[pbor]

👍