hz.tools/fftw: hz.tools/sdr/fft bindings for libfftw3

The hz.tools/fftw package contains bindings to libfftw3 to implement the hz.tools/sdr/fft.Planner type to transform between the time and frequency domain.

The git repo can be found at github.com/hztools/go-fftw, and is importable as hz.tools/fftw.

This is the default throughout most of my codebase, although that default is only expressed at the “leaf” package – libraries should not be hardcoding the use of this library in favor of taking an fft.Planner, unless it’s used as part of testing. There are a bunch of ways to do an FFT out there, things like clFFT or a pure-go FFT implementation could be plugged in depending on what’s being solved for.