Switch to realfft (and rustfft) from fftw

[hombit]

realfft/rustfft provide better performance than FFTW on every platform I tested (see benchmarks in this repo, n >> 1024). The only exception is FFTW with the MKL backend, which is significantly faster on Intel CPUs. Additionally, FFTW is one of the two libraries (the other being GSL) that cause this project to be GPL licensed. Also it'd make our Python wheel build pipeline much simpler.