Experimentally integrate C++ Witness Generator

[oskarth]

Problem

See #109

Suggested approach

Suggested course of action: We try for C++ witness gen as experimental feature and create a shared branch?PR in mopro for doing an MVP integration, both Vivian and Florent can collab on it together there. The branch/PR can be ugly and breaking and we can clean it up later.

Once it works in MVP form it should be trivial to then integrate with proof-of-passport. This would also allow us to work together on it and sharing code/effort

Details

• https://github.com/0xPolygonID/witnesscalc
• https://github.com/rarimo/FreedomTool
• https://github.com/rarimo/FreedomToolAndroid

Also relevant: #44

Acceptance criteria

• Understanding in diff in terms of interface when using C++ version (e.g. require rapidsnark?)
• C++ witness generator integrated and working end to end (dirty)
• Integrated with feature flag cleanly

[oskarth]

cc @vivianjeng and @0xturboblitz

[0xturboblitz]

I was able to use it on this branch to calculate a witness for the multiplier circuit. Now I have no idea how to feed it to arkworks' prover, as it comes out as bytes and arkwork's is structured. The most logical path would be to then use rapidsnark as it's part of the same stack, just like the FreedomTools people are doing here, but now this is becoming quite a different stack.
I can move that to the mopro repo too if it helps

[oskarth]

https://github.com/rust-lang/rust-bindgen can probably be used for this. If it is using some C++ template stuff, might have to have C intermediate representation