Improve and clarify degree-based order implementation

[gdalle]

When we shrink a bucket, we don't need to write v into the spot we remove

This implementation will never be equivalent to ColPack because we can't allocate at the ends of buckets whenever we want

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b465a79) to head (90889ed).
Report is 1 commits behind head on main.

Additional details and impacted files

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##              main      #223   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           15        15           
  Lines         1720      1716    -4     
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- Hits          1720      1716    -4     

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