Provide same features as RustFFT, mapping to them one-to-one

[miikkas]

I noticed that in projects depending separately on both RealFFT and RustFFT, the default features of RustFFT get selected even when configuring that dependency with default-features = false and features = ["avx"].

This PR adds the features available in RustFFT to RealFFT as well, enabling the corresponding RustFFT features only when selected. RealFFT's default feature enables RustFFT's default feature, so existing use cases shouldn't be affected.

[HEnquist]

Thanks for the PR! This is a nice improvement, I'll include it in the next release. I expect a new release of RustFFT very soon (see ejmahler/RustFFT#157), I'll just wait for that before making a new release here.

One little point of feedback. I think the readme could mention that the features are only passed on to RustFFT and don't affect RealFFT itself at all.

[miikkas]

That's a good point. I have modified the added documentation bit according to your feedback now.

[HEnquist]

Thanks, looks good! I'll merge this when it's time to create the next release. I prefer to keep the master branch identical to the latest release on crates.io to avoid some confusion.

[ejmahler]

Rustfft update published: https://crates.io/crates/rustfft/6.4.0

[HEnquist]

RealFFT updated too :)
https://crates.io/crates/realfft/3.5.0