Implement constraint importer from gnark for usage within Groth16 prover

[gabrielfior]

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As raised in this comment, it would be great if we could validate the algorithms with intermediate results from other libraries, such as gnark.

The idea would be: -> Implement a CSV importer of R1CS (for example, https://play.gnark.io/ outputs such a CSV representing the constraints, select cubic equation on the top right dropdown) -> Convert said R1CS into a QAP representation, that we can then forward to the Groth16 prover -> Write a test for said conversion, proving and verification