Improve API for circuit compression

[moCello]

Resolves #804

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (db46f87) 85.74% compared to head (4969419) 85.71%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #807      +/-   ##
==========================================
- Coverage   85.74%   85.71%   -0.04%     
==========================================
  Files          57       57              
  Lines        4091     4095       +4     
==========================================
+ Hits         3508     3510       +2     
- Misses        583      585       +2     

Files	Coverage Δ
src/compiler.rs	99.44% <100.00%> (ø)
src/composer/compress.rs	85.95% <100.00%> (+0.12%)	⬆️
tests/composer.rs	96.49% <100.00%> (+0.06%)	⬆️
src/composer/circuit.rs	45.45% <83.33%> (+45.45%)	⬆️
src/composer.rs	95.04% <33.33%> (-0.62%)	⬇️

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[HDauven]

LGTM