Add Vec3::rotate_towards

[tim-blackbird]

Added Vec3::rotate_towards with identical behavior to Vec2::rotate_towards

I added some extra tests to Vec2::rotate_towards as well, and adjusted the docs to be more accurate since the method does not interpolate the lengths of the vectors.

I noticed another issue with that. Vec2::rotate_towards returned rhs when the angle is too small. This means that only in that case the length of the resulting vector is equal to that of rhs instead of self.
It also differs from Quat::rotate_towards which returns self when the angle is too small, so I've updated it to return self instead.

[bitshifter]

I think perhaps returning the rhs when the angle is too small in both this and the quat version might be a better option.