[opus] [Opus][RFC][FFT][Fixed-Point][NEON] NEON-Optimize Fixed-Point FFT?

[Timothy B. Terriberry]

Phil Wang wrote:
> Is it int32 good enough? Benchmark data shows that FFT using int16 is much faster than FFT using int32.

For kissfft, we found that we needed the input and output to be 32 bits 
to get enough accuracy, but that the twiddles could be 16-bit.