OpenVM: verifying an arithmetic circuit example

[clarus]

The goal is to verify an example of a circuit from OpenVM: https://github.com/openvm-org/openvm

This is a RISC-V zkVM with an open instruction-set format. For now, it implements RISC-V 32 bits plus some pre-compiles, but the idea is that it could also implement MIPS, WASM, ... if implemented. The whitepaper is here: https://openvm.dev/whitepaper.pdf

They have a custom version of Plonky3. Something that is new compared to what we have looked for Blake3/Keccak is the idea of "bus" and "lookup-argument". They also need to respect some global invariants on the Program Counter or handling of RAM memory for each sub-circuit.

The folder they suggested we verify is Branch Eq: https://github.com/openvm-org/openvm/tree/main/extensions/rv32im/circuit/src#2-branch-eq