Function immutable_seq::digit::add_3_digits

pub fn add_3_digits<T, M>(d0: Digit<T, M>,
                          d1: Digit<T, M>,
                          d2: Digit<T, M>)
                          -> Digit<T, M> where T: Measure<M> + 'static, M: Add<Output=M> + Copy + 'static