improve performance of hessians with static arrays

[longemen3000]

by adding the suggestion done in #720 .

Closes #720

[DNF2]

Is there any chance of this being merged? It has a significant impact on my library.

[devmotion]

All tests are failing currently.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.54%. Comparing base (42e0aa6) to head (a6306bf).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
- Coverage   89.82%   89.54%   -0.29%     
==========================================
  Files          11       11              
  Lines        1032     1033       +1     
==========================================
- Hits          927      925       -2     
- Misses        105      108       +3     

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