常量格式化

一个简单的宏，使concat!更易用/更易于阅读。

use static_format::const_format;

macro_rules! period {
    () => {'.'}
}

fn main() {
    let formatted = const_format!(
        "{}, there are {} formatted {}{}",
        "Hello", 4, "arguments", period!()
    );
    assert_eq!(formatted, "Hello, there are 4 formatted arguments.");
}