cff backend

[cff](https://github.com/defi-formal/cff) ([paper](https://eprint.iacr.org/2021/1147)), is a framework and toolchain that allows for the formal analysis of economic properties. It appears to be a remakrably powerful tool, quoting from the abstract: "it can automatically and mechanically extract all possible economic attacks on users' cryptocurrency across modeled contracts".

cff is implemented in K, we should build a backend that allows us to compile an act spec directly into a cff compatible model in K.

Start with the cff repository and linked paper, then determine how an act spec is represented in this repository. Identify the K output required for a cff-compatible model and establish how compilation will be verified; the issue names no files or tests.

Written by the indexing model from the issue text.