Benchmarks for ZK proofs

[dvdplm]

Adds a set of benchmarks for our ZK proofs, starting with FacProof and AffG.

Also contains a synthetic benchmark that measures the performance of exponentiations with large exponents vs reducing such exponents by the totient.

Given the proof code is not part of the public API (and shouldn't be), this PR introduces a "private_benches" feature that allows a thin(-ish) criterion benchmark wrapper ("benches/zk_proofs.rs") to call code in src/private_benches/mod.rs. Here we have access to everything that is pub(crate).

Run the benchmarks with cargo bench --bench zk_proofs --features private_benches.

The PR also initialize tracing during the benchmarks, which can be useful when doing optimization work.

[codecov]

Codecov Report

Attention: Patch coverage is 0.14815% with 674 lines in your changes missing coverage. Please review.

Project coverage is 84.33%. Comparing base (423f1b5) to head (7cf86ec).

Files with missing lines	Patch %	Lines
synedrion/src/private_benches.rs	0.00%	673 Missing ⚠️
synedrion/src/cggmp21/sigma/fac.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   93.06%   84.33%   -8.74%     
==========================================
  Files          36       36              
  Lines        7012     7684     +672     
==========================================
- Hits         6526     6480      -46     
- Misses        486     1204     +718     

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