PoC: fused compute kernels

[Shnatsel]

This is a naive way to perform f64 DIT for chunk sizes 2 and 4 in one function, removing the load/store overhead between these two stages since the whole thing can be kept in registers for the duration of the operation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.96%. Comparing base (52322d0) to head (46787b0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   99.82%   98.96%   -0.86%     
==========================================
  Files          13       13              
  Lines        2281     2319      +38     
==========================================
+ Hits         2277     2295      +18     
- Misses          4       24      +20     

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