Proof generation parallelization

[0xmad]

As a user I'd like to parallelize proof generation to improve speed of overall process.

• Research if it's possible to use workers for proof generation: better to use promise all because proof generation groth16 is already implemented worker logic and doesn't make sense to wrap it in a web worker again. Plus, using I/O operations is optimized better than trying to put it into a web worker
• Try to generate different proofs with forking and then combine the results