Add proof input preprocessing

[rozbb]

Description

If a verifier knows the public inputs to a circuit, it can now preprocess them in advance. This is a useful trick for reducing the online time of protocols which require a verification.

This does not implement the same functionality for circuits. This is because the circuit is implemented as a SNARKGadget, which does not expose input preprocessing methods. I'm not certain if all the NIZK schemes in arkworks have the ability to do this trick, so it might not be possible to extend SNARKGadget.

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• Wrote unit tests
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• Updated relevant documentation in the code
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[ValarDragon]

LGTM