Allow out-of-order FFT

[weikengchen]

The codebase has implementations of the FFT. However, throughout the public interface, one can only use the "II" mode, in-order input, and in-order output. OI and IO are basically not used in the entire system.

This is not suitable for some IFFT that prefers an in-order input, and out-of-order output, and then immediately performs some other operations and then FFT it back.

Proposal: relax the IO/OI interfaces to be "pub" in radix2 so people can use them. Particularly, note that users do not even have access to the enum "fft::FFTOrder"

[weikengchen]

Let me know if we want to move ahead with it.

[weikengchen]

ADD: one can also bypass this limitation by preprocessing the data, e.g., perform a bit reversal to cancel the other bit reversal implemented in arkworks-rs.

[Pratyush]

I think we can accept it, but the issue is that we don't have support for that on the multi-radix FFTs (AFAIK). I think if we have a good solution there then it makes sense.

[weikengchen]

I think we will leave it for the release after v0.4.0