Add a fuzzer for the new, much more complex FFT implementation

[Shnatsel]

No description provided.

[Shnatsel]

No issues after 10 million executions.

I've run it without overflow checks and debug assertions, however. I was only interested in release-mode panics and memory safety issues.

[pdeljanov]

I've reworked the rustfft integration in master so that it's only used internally by the Imdct. This is because using it for Fft forces a SemVer breaking change. In v0.6 I'll switch Fft over.

Aside from that, don't you think fuzzing rustfft should be done in the rustfft repo?

[Shnatsel]

I'm working on fuzzing in the RustFFT repo, yes: ejmahler/RustFFT#110

But since you said that Symphonia uses RustFFT in specific and limited ways, I thought it might be a good idea to fuzz that codepath first, which should be simpler than the general case.

The RustFFT APIs are more complicated and I still haven't built a working fuzzer in the upstream repo. I could move this code there, if you wish.