Make `realfft` an optional dependency?

[emilk]

This looks like a really nice crate, but I was wondering if it would be possible to make realfft an optional dependency, for users that only want/need the fast/low-quality resampling? Or are all paths using the fft?

[HEnquist]

It would be no problem to make the FFT resamplers optional. The fast ones don't use the FFT and are pretty small, so omitting the FFT should reduce binary size and compile time nicely.

[HEnquist]

see #78