Faster RP commits with multi-exponentiation

[dvdplm]

Use the MultiExponentiationBoundedExp trait to speed up commitment creation, which is common operation in most of our ZK Proofs.

Note: Builds upon #173

[codecov]

Codecov Report

Attention: Patch coverage is 13.01546% with 675 lines in your changes missing coverage. Please review.

Project coverage is 84.41%. Comparing base (a3bbfc6) to head (80c49a3).

Files with missing lines	Patch %	Lines
synedrion/src/private_benches.rs	0.00%	673 Missing ⚠️
synedrion/src/cggmp21/sigma/aff_g.rs	85.71%	1 Missing ⚠️
synedrion/src/cggmp21/sigma/fac.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   92.46%   84.41%   -8.06%     
==========================================
  Files          35       36       +1     
  Lines        7030     7723     +693     
==========================================
+ Hits         6500     6519      +19     
- Misses        530     1204     +674     

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