Allow angle and/or matrix to be cached in `Rotation2` about nalgebra

[sebcrozet]

Computing the matrix on-the-fly based on the angle would make applying the rotation to a point/vector quite expensive. Storing the matrix sin/cos in Rotation2 is a deliberate choice to optimize for this particular use-case. If benchmarks show that there is a need to cache this angle value, then it makes sense to leave it up to the user.

[edgimar]

@sebcrozet, I'm not sure if I understand your point correctly. It would seem that if you start with an angle, and want to apply the rotation to a point/vector, then you would need to pay the cost of converting to sin/cos at some point (either when you create the Rotation2 instance, or when you apply the rotation). It can also be the case that you want to work only with the angle, and don't ever need to compute the sin/cos values. The point is to lazily compute (and cache) either angle or sin/cos as needed, but to avoid computing it if not needed.

Let me know if it would be helpful / worthwhile to submit a PR that demonstrates the idea.