Improve SYMKey init (allow setting k and/or key)

[jschlyter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.46%. Comparing base (e95fb93) to head (3092472).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   76.43%   76.46%   +0.02%     
==========================================
  Files          44       44              
  Lines        4550     4554       +4     
  Branches      859      861       +2     
==========================================
+ Hits         3478     3482       +4     
  Misses        785      785              
  Partials      287      287              

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