Preparing pil2-stark-js to be ported to C++ prover

[xavi-pinsach]

TODO List (add all tasks pending)

• [X] Generate a STARK from PIL2 with one subproof and one air
• [X] Make pil2 stark tests (simple+fibonacci) work in this repo
• [X] Integrate PIL verify as a "debug" flag
• [X] Integrate basic verifiers circom circuits in tests

[RogerTaule]

• [X] Adapting prover to be ported to C++
• [ ] Integrating new parser (basic version done, pending to add subproof values and upgrade expressions binary)
• [ ] Working on expressions and hints