riscv-formal proofs for SErial RiscV (SERV)

Quickstart guide:

First install Yosys, SymbiYosys, and the solvers. See here for instructions. Then build the version of SERV with RVFI support and riscv-tools, and generate the formal checks:

bash generate.sh

Then run the formal checks:

make -C checks -j$(nproc)
bash cexdata.sh