Add Default impl for MutexNode

[pedromfedricci]

Default trait implementation simply calls Self::new().

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.98%. Comparing base (c877dd4) to head (f568e2c).
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   96.95%   96.98%   +0.02%     
==========================================
  Files           5        5              
  Lines         230      232       +2     
==========================================
+ Hits          223      225       +2     
  Misses          7        7              

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