Add new constructors for Circle and Sphere

[Jondolf]

Objective

Make APIs more consistent and ergonomic by adding a new constructor for Circle and Sphere.

This could be seen as a redundant "trivial constructor", but in practise, it seems valuable to me. I have lots of cases where formatting becomes ugly because of the lack of a constructor, like this:

Circle {
    radius: self.radius(),
}
.contains_local_point(centered_pt)

With new, it'd be formatted much nicer:

Circle::new(self.radius()).contains_local_point(centered_pt)

Of course, this is just one example, but my circle/sphere definitions very frequently span three or more lines when they could fit on one.

Adding new also increases consistency. Ellipse has new already, and so does the mesh version of Circle.

Solution

Add a new constructor for Circle and Sphere.

[IQuick143]

Seems like a good straightforward change.

[Kanabenki]

Very small doc changes to match the other doc comments, otherwise looks good!

Edit: forgot to expand the files so I didn't see that some other doc comments in the file weren't ending phrases with a dot, IMO we should always add one for consistency but there doesn't seems to be fixed guideline for that at the moment, so feel free to ignore if you want.

[Kanabenki]

Suggested change

	/// Create a new [`Circle`] from a `radius`
	/// Create a new [`Circle`] from a `radius`.

[Jondolf]

Most constructors (Plane2d::new, Segment2d::new, Polyline2d::new, Triangle2d::new, Rectangle::new...) in these files currently don't end the doc comment with a period unless the comment spans multiple lines. The doc comments for Circle and its radius also don't end with a period. I'd like to change them all to end with a period and be more consistent though.

[Kanabenki]

Suggested change

	/// Create a new [`Sphere`] from a `radius`
	/// Create a new [`Sphere`] from a `radius`.