Speedup coset fft

[ValarDragon]

Description

Speeds up coset FFTs by N field multiplications. It does this by combining the final multiplication by |D|^{-1} with the coset shift

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[ValarDragon]

Putting allow(unused) here, since we want to benchmarking changing roots of unity with this function

[Pratyush]

LGTM modulo the duplication comment

[ValarDragon]

Benchmarked on snarky. This appears to apply a speedup of approximately 5%. (Started benchmark at 2^15, where it was 5.5%. At 2^20 its 4.25%)

[Pratyush]

That's awesome =)