Macro pair! to create tuple with Some

---

Seen in the sqlite crate.

examples/macros/macro-sqlite-pair/src/main.rs

macro_rules! pair(
    ($one:expr, $two:expr) => (($one, Some($two)));
);

fn main() {
    let a = pair!("id", "1");
    assert_eq!(a, ("id", Some("1")));
}