C backend for PocketFFT

[Vindaar]

Adds C backend support for PocketFFT. Uses the C library directly and builds it statically with the Nim project.

Selection of the appropriate backend for C or C++ targets is done in impulse/fft/pocketfft.nim automatically. It's maybe not fully ideal, because their APIs are not a drop in replacement for the other.

[AngelEzquerra]

Adds C backend support for PocketFFT. Uses the C library directly and builds it statically with the Nim project.

Selection of the appropriate backend for C or C++ targets is done in impulse/fft/pocketfft.nim automatically. It's maybe not fully ideal, because their APIs are not a drop in replacement for the other.

This is really great! I tried the new C backend on Windows and it seems to work find (and gives the same results as numpy.fft in the few small tests I did).

As we discussed, ideally this would be moved up a level so that users can just import impulse/fft.

In a future PR we could add support for the same normalization options supported by numpy. We could also add a separate real_fft that takes and outputs real values, while the regular fft should always return complex values.