Stabilize LazyCell and LazyLock

[pitaj]

Closes #109736

This stabilizes the LazyLock and LazyCell types:

static HASHMAP: LazyLock<HashMap<i32, String>> = LazyLock::new(|| {
    println!("initializing");
    let mut m = HashMap::new();
    m.insert(13, "Spica".to_string());
    m.insert(74, "Hoyten".to_string());
    m
});

let lazy: LazyCell<i32> = LazyCell::new(|| {
    println!("initializing");
    92
});

r? libs-api