Function calculate_z

pub fn calculate_z(x: &[RqVector], random_c: &RqVector) -> RqVector

calculate z = c_1s_1 + … + c_rs_r or calculate Az = c_1t_1 + … + c_rt_r

@param: x: witness s_i or Ajtai commitments t_i @param: random_c: c_i from challenge set

return z