R2DH: use RegularizedExecutionStats instead of Generic

[MaxenceGollier]

closes #198

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.15%. Comparing base (e0f214d) to head (41c09e1).
⚠️ Report is 188 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #235       +/-   ##
===========================================
+ Coverage   61.53%   86.15%   +24.62%     
===========================================
  Files          11       13        +2     
  Lines        1292     1575      +283     
===========================================
+ Hits          795     1357      +562     
+ Misses        497      218      -279     

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