Integrate RISCV-formal verification framework

[TheDeepestSpace]

Research RISCV-formal. Ideally we want to integrate it into our design similarly to how we did with RISCOF in #38

[TheDeepestSpace]

It looks like we would need to implement RVFI interface to use this framework. If we do this for multi-cycle implementation now, how hard would it be to port to pipelined implementation?