reimFFT pretty slow

[lpatiny]

reimFFT is a limiting step in some of the processing. However I tried to copy the original library, used typed array and try some other improvements without success. I have the impression that is library is really very well optimized.

However while the speed is optimal the current memory management may not be optimal.

1. Does it make sense to have an in-place (option inPlace: true) modification of the data ? at the end we create a new reimData but we could reuse the original one.

2. If we have 2D we recreate a new FFT that creates internally an array with cos and sin (pretty long to calculate). https://github.com/indutny/fft.js/blob/master/lib/fft.js. It think this method could accept as an option a fft instance (could throw if size is not the expected size).