Turn Composer trait into a struct

[moCello]

Resolves: #802

[codecov]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (4b3568f) 85.63% compared to head (08dc0f6) 85.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
+ Coverage   85.63%   85.66%   +0.03%     
==========================================
  Files          57       57              
  Lines        4128     4095      -33     
==========================================
- Hits         3535     3508      -27     
+ Misses        593      587       -6     

Files	Coverage Δ
src/composer/compiler.rs	99.44% <100.00%> (ø)
src/composer/gate.rs	0.00% <ø> (ø)
src/composer/prover.rs	95.54% <100.00%> (ø)
src/composer/verifier.rs	86.00% <100.00%> (ø)
src/constraint_system/constraint.rs	61.01% <ø> (-9.48%)	⬇️
src/proof_system/proof.rs	95.94% <ø> (ø)
src/runtime.rs	21.42% <ø> (+4.76%)	⬆️
tests/append_gate.rs	93.75% <100.00%> (ø)
tests/composer.rs	96.42% <100.00%> (+0.27%)	⬆️
tests/decomposition.rs	87.75% <100.00%> (-4.41%)	⬇️
... and 14 more

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4b3568f...08dc0f6. Read the comment docs.

[xevisalle]

LGTM