Hello World macro

---

• The name of the macro is hello_world
• The () matches empty input.

examples/macros/hello-world/src/main.rs

macro_rules! hello_world {
    () => {
        println!("Hello World!");
    };
}

fn main() {
    hello_world!();

    // hello_world!(apple);
    // hello_world!("apple");
    // ^ no rules expected this token in macro call
}