Plonk: Add solidity verifier

[georgwiese]

The main changes are:

• zokrates_bellman:
  • plonk.rs: serialize_vk() now pre-computes omega. This is redundant, but needed to render the solidity verifier.
• zokrates_cli:
  • tests/integration.rs: Adjusted the test_compile_and_witness_dir test to run only for plonk. It should pass now, testing the full pipeline, including proof verification in solidity.
• zokrates_proof_systems:
  • Added solidity_renderer.rs, which is an adjusted version from this code from matter-labs.
    • See this link for a comparison between 037ec0a (when I copied the original file from the matter-labs repo) and the current head of georgwiese:plonk-add-verifier.
  • PlonkVerifier.sol: The Plonk solidity template, copied and adjusted from this code from matter-labs.
  • scheme/plonk.rs: This file moved to the zokrates_bellman create, as mentioned above.
  • to_token.rs: Some refactorings
• Removed some unused imports.

To test, run:

$ cargo test --package zokrates_cli -- --nocapture test_compile_and_witness_dir --ignored

[Schaeff]

Just a few small things! 👍
We can fix CI on the target branch I think

[georgwiese]

Thanks for the review!

The integration test fails now, as it did before this PR. I'll try to fix tests in the next PR, which is about splitting the zokrates_bellman crate to support both bellman versions.