Keep witness argument immutable on halo2_mock_prover

[sraver]

Closes #170

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On the SuperCircuit's halo2_mock_prover method, we are currently overriding the super_witness dict object.
Initially it holds the TraceWitness objects of each internal circuit, but after the loop it holds the JSON representation of the witness of the circuit.

Since the elements in the dict are indexed by their rust_id, after the loop all the elements have been replaced, leaving the caller of halo2_mock_prover with a "modified" witness.

It's not a big problem, but it's confusing sometimes when writing tests, if you want to verify some of the assignments, because the order of lines becomes relevant.

I didn't add the return of the witness_json because so far I've seen no use for it. Maybe it could be interesting.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f1e7491) 49.67% compared to head (29ba210) 49.67%.

❗ Current head 29ba210 differs from pull request most recent head ba932e5. Consider uploading reports for the commit ba932e5 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #175   +/-   ##
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  Coverage   49.67%   49.67%           
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  Files          21       21           
  Lines        5751     5751           
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  Hits         2857     2857           
  Misses       2894     2894           

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[qwang98]

LGTM and thanks for suggesting this change! We don't need to return witness JSON as it's only used to serialize a Python object to be parsed by Rust.

[qwang98]

@sraver I'd just request that you change the version number below to the format [[0.1.YYYYMMDDVV]] where [[VV]] can be 00 if it's the first pull request we merge to the main on a day. Only this will trigger the update for the published Python package.

https://github.com/privacy-scaling-explorations/chiquito/blob/main/Cargo.toml#L3

After that, feel free to merge. :)

[sraver]

@qwang98, just updated the version.

I can't merge tho, please proceed for me ^_^