Add copy method to cstruct instance

[Schamper]

Closes #79

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.06%. Comparing base (194b1b5) to head (cc23c7a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
+ Coverage   93.01%   93.06%   +0.05%     
==========================================
  Files          21       21              
  Lines        2435     2453      +18     
==========================================
+ Hits         2265     2283      +18     
  Misses        170      170              

Flag	Coverage Δ
unittests	93.06% <100.00%> (+0.05%)	⬆️

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