Rotation api extension

[RobWalt]

Objective

• Another way of specifying rotations was requested in Angle type #11132 (comment)

Solution

• Add methods on Rot2
  • turn_fraction(fraction: f32) -> Self
  • as_turn_fraction(self) -> f32
• Also add some documentation on range of rotation

Testing

• extended existing tests
• added new tests

Showcase

let rotation1 = Rot2::degrees(90.0);
let rotation2 = Rot2::turn_fraction(0.25);

// rotations should be equal
assert_relative_eq!(rotation1, rotation2);

// The rotation should be 90 degrees
assert_relative_eq!(rotation2.as_radians(), FRAC_PI_2);
assert_relative_eq!(rotation2.as_degrees(), 90.0);

[Jondolf]

LGTM! I think this is a useful API. Left some non-blocking suggestions

[Jondolf]

CC @johannesvollmer; this doesn't fully address what you described in #11132 (comment), because Rot2 is a unit complex number that should always be normalized, so it can't represent turns greater than a full rotation, and the internal representation still isn't turns. But at the API level, I think this is still useful for rotations.

[johannesvollmer]

Yes, still useful :) nice

[janhohenheim]

Nice! Left some minor documentation improvements :)
IMO the "resulting rotation" in the docs would be clearer as the "input rotation", since otherwise it reads as if the resulting rotation was once in radians, once in degrees, and once in fractions, which is not the case. But I'll leave that up to you.