Add Zeroize trait implementation for Witness

[moCello]

Resolves #818

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.32%. Comparing base (c79dc3b) to head (8957552).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #820   +/-   ##
=======================================
  Coverage   85.32%   85.32%           
=======================================
  Files          57       57           
  Lines        3687     3687           
=======================================
  Hits         3146     3146           
  Misses        541      541           

Files	Coverage Δ
src/composer/constraint_system/witness.rs	66.66% <ø> (ø)

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[ZER0]

LGTM, just small nits to fix; I will approve once fixed.

[ZER0]

Thanks for addressing the comments!