Feature: FFT support

[aliceisjustplaying]

Oh my god we finally got here.

A few comments:

• The feature is hidden behind the --fft flag. If it's not enabled or we're on unsupported platforms (3DS, baremetal PI, Android) the functions fft() and ffts() simply return 0 and nothing gets even initialized.
• Took the liberty to add the authors of the past Byte Battle / FFT patches to the contributors as well as the prior work in @TheNuSan's Bonzomatic fork. Let me know if you'd like any changes regarding that
• Had to squash most of the FFT feature into one commit because rebasing became nigh-impossible after some recent changes
• Passing around fftdata.h is kind of gnarly but it's the least bad solution I've found. If you have ideas for improvements they're extremely welcome
• This feature will definitely keep evolving, but this is a good moment to merge it to upstream and iterate from there

[nesbox]

@aliceisjustplaying Awesome, thank you 👍