Several related improvements to our Fast Fourier Transform
implementation:
- FFT now operates on spans, allowing it to use many more container
types other than Vector. It's intended anyways that FFT transmutes the
input data.
- FFT is now constexpr, moving the implementation to the header and
removing the cpp file. This means that if we have static collections
of samples, we can transform them at compile time.
- sample_data.data() weirdness is now gone.
LibDSP can greatly benefit from this nice FFT implementation, so let's
move it into the fitting library :^)
Note that this now requires linking SoundPlayer against LibDSP. That's
not an issue (LibDSP is rather small currently anyways), as we can
probably make great use of it in the future anyways.
