Proof Rules: Add automatic support for iid-rules

[Philipp15b]

We should implement support for the iid proof rules presented in this paper: https://link.springer.com/chapter/10.1007/978-3-319-89884-1_7. Theorem 3 gives a closed form for the loop's wp and Theorem 4 gives a closed form for ert.

The implementation needs to be at the vc generation level since the closed forms require access to the current post.