PoC: Radix 2^2 + parallel kernel

[Shnatsel]

Stacked on top of #103

Adds parallelism to the FFT kernel scaffold, which lets us parallelize the final few stages that aren't parallelized by recursion.

Recursion is still preferable because it provides natural cache locality and makes the algorithm cache-oblivious.

Performance vs main: -27% time taken on Zen4, -38% time taken on M4
measured on cargo run --profile=profiling --all-features --example benchmark -- 64 28 5

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.24038% with 103 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.15%. Comparing base (3fc78e0) to head (bdf8335).

Files with missing lines	Patch %	Lines
src/kernels/dit.rs	66.44%	100 Missing ⚠️
src/algorithms/dit.rs	97.45%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   99.79%   94.15%   -5.65%     
==========================================
  Files           8        8              
  Lines        1438     1812     +374     
==========================================
+ Hits         1435     1706     +271     
- Misses          3      106     +103     

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