Interface Digit

src/helper.rs:12–14

Abstract Digit trait, fill the empty space to specific length.

mod digit {
    /// Abstract Digit trait, fill the empty space to specific length.
    pub trait Digit<T> {
        fn digit(self, d: T) -> String;
    }

    impl Digit<i32> for i32 {
        fn digit(self, d: i32) -> String {