Fix issue #228

[tcbrindle]

Manually implement set_symmetric_difference_adaptor::cursor_type's operator== so that it doesn't compare the state_ field

Fixes #228

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.29%. Comparing base (f539c11) to head (a4b9bb6).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
- Coverage   98.60%   98.29%   -0.32%     
==========================================
  Files          69       69              
  Lines        2578     2632      +54     
==========================================
+ Hits         2542     2587      +45     
- Misses         36       45       +9     

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