Function build_eq_x_r 

pub fn build_eq_x_r<C: SemiringConfig>(
    cfg: &C,
    r: &[C::Element],
) -> Result<DenseMultilinearExtension<C::Element>, ArithErrors>

This function build the eq(x, r) polynomial for any given r.

Evaluate eq(x,y) = \prod_i=1^num_var (x_i * y_i + (1-x_i)(1-y_i)) over r, which is eq(x,y) = \prod_i=1^num_var (x_i * r_i + (1-x_i)(1-r_i))