Add Default trait implementation for Witness

[moCello]

Resolves #815

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.32%. Comparing base (7505e9a) to head (7312fa9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #816      +/-   ##
==========================================
- Coverage   85.34%   85.32%   -0.03%     
==========================================
  Files          57       57              
  Lines        3686     3687       +1     
==========================================
  Hits         3146     3146              
- Misses        540      541       +1     

Files	Coverage Δ
src/composer/constraint_system/witness.rs	66.66% <0.00%> (-6.07%)	⬇️

---

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 7505e9a...7312fa9. Read the comment docs.