Flow: New formal verification flow using fvm-tool

[ben-marshall]

The current formal flow is not scaling. Some instructions are taking >40 hours to prove. This is because multiple assertions are active at once, or single assertions are trying to express too much.

I've created a basic tool which makes generating parts of the formal environment easier, and lets one split groups of assertions into individual ones. We can then run more, smaller proof jobs which will hopefully complete quicker.

The tasks here are to:

• [ ] Copy the existing assertions into the new fvm-tool format.
• [ ] Add a new flow/fvm/ directory and make flow which will automate everything.
• [ ] Add a flow section which uses Yosys to generate the N required testbenches, one for each assertion, in SMT2 format.
• [ ] Add a flow section which can run one or many of the proofs in parallel, initially using the make -j flag.

[ben-marshall]

Closing issue for now since the work was taking too long and providing to small an improvment to be useful.

• Continuing with the old flow and will optimise individual proofs as needed.