Remember registered user-operations and free them on exit

[andrsd]

Closes #55

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.28%. Comparing base (a20ea6e) to head (419a1c0).
⚠️ Report is 2 commits behind head on 3.0.x.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##            3.0.x      #97   +/-   ##
=======================================
  Coverage   99.28%   99.28%           
=======================================
  Files          14       14           
  Lines         559      563    +4     
=======================================
+ Hits          555      559    +4     
  Misses          4        4           

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