Use the radix-4 FFT for odd powers of two

[andreasnoack]

This seems to cut the runtime for odd powers of two by more the two.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.96%. Comparing base (ba40c9c) to head (bc7ed03).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
- Coverage   98.04%   97.96%   -0.08%     
==========================================
  Files           4        4              
  Lines         409      393      -16     
==========================================
- Hits          401      385      -16     
  Misses          8        8              

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[dannys4]

Happy to approve, but at this point do you think we should just kill the general pow2 code since this subsumes?

[andreasnoack]

I actually did that in a first version but then added it back in because I wasn't sure if there'd be situations where it would be a better choice. Let me drop it as it reduces the code. If we need it again at some point, it can easily be recovered.