Modularize prover-cpp

[leonardoalt]

Tracking powdr-labs/zkevm-prover#3 on the powdr side as well.

[leonardoalt]

This is now paused in favor of #15 (comment)

[leonardoalt]

@lvella is working on this again with #1211, #1193, and https://github.com/powdr-labs/pil-stark-prover/

[lvella]

I don't think this issue is still valid, now that we have the wrapper crate powdr-labs/pil-stark-prover. Also, linking prover-cpp instead of calling it as an external program wouldn't help much if we still need the node.js stuff from pil-stark.