Faster jtprod if dense output

[blegat]

Needed for kocvara/Loraine.jl#29

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.40%. Comparing base (083af50) to head (997bdea).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   99.39%   99.40%   +0.01%     
==========================================
  Files          27       27              
  Lines        1312     1337      +25     
==========================================
+ Hits         1304     1329      +25     
  Misses          8        8              

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