Update the structure TreeSet

[amontoison]

Related to #68

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.81%. Comparing base (cb1c2c6) to head (c8400b7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   99.83%   99.81%   -0.02%     
==========================================
  Files          11       10       -1     
  Lines         605      552      -53     
==========================================
- Hits          604      551      -53     
  Misses          1        1              

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[gdalle]

This is superseded by the result type refactor in #71.
I put the complexity in the result constructors rather than the TreeSet, that way we can still have a fast underlying coloring if we want to.