Refactor polymorphic keys method

[tagliala]

Use select instead of map.compact to:

• Eliminate unnecessary object creation and array compaction
• Maintain identical functionality
• Improve readability and performance

Comparison (IPS):
                keys:  1716164.8 i/s
            keys_old:  1531182.7 i/s - 1.12x  (± 0.00) slower

Comparison (Memory):
                keys:        120 allocated
            keys_old:        160 allocated - 1.33x more

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.92%. Comparing base (1aeb754) to head (17e48bd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #941   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files          14       14           
  Lines         557      557           
=======================================
  Hits          551      551           
  Misses          6        6           

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