feat(nifs): optimize compute_G

[chaosma]

The goal of compute_G is to get evaluations of G(X) over coset domain so that we can quickly get evaluations of K(X) and use ifft to get its coefficient form.

In current implementation, we first get coefficient form of G(X), and then do coset_fft for G(X) to get its evaluations. However, this part can be avoided by directly return evaluations of G(X) over coset domain instead of coefficient form of G(X). This will save us two (small-sized) FFT.

[cyphersnake]

The whole point is to omit the ifft at the end and return calculation by itself?

[cyphersnake]

We don't know the degree of G(X) without `ifft'. @chaosma, any idea how to find it by calculated G(X) points from the coset cyclic group?