Add KFAC

[SamDuffield]

Would be awesome to add KFAC support within posteriors, would take some planning though as we'd like to implement it in a general purpose way and KFAC (unlike the other posteriors methods explicitly uses the layerwise structure of neural networks).

• KFAC Optimization https://arxiv.org/abs/1503.05671
• KFAC Laplace https://openreview.net/forum?id=Skdvd2xAZ
• KFAC VI/EKF? https://arxiv.org/abs/1712.02390