Outsource all KFAC computations to `curvlinops`

Once KFAC is part of a [`curvlinops`](https://github.com/f-dangel/curvlinops/) release, we can try to remove all KFAC related computations from this repository and use `curvlinops` instead. Currently this is not trivially possible, but I think it would be really valuable to move to this solution at some point, since it allows us to only maintain one KFAC implementation that other methods and projects can use.