Function power2round_vec 

pub fn power2round_vec(
    t: &[[i32; 256]],
    len: usize,
) -> ([[i32; 256]; 8], [[i32; 256]; 8])

Apply power2round to every coefficient of a polynomial vector.

Returns (t1, t0) as fixed arrays where each element satisfies t[i][j] = t1[i][j] * 2^d + t0[i][j].