Stabilize `let_chains` in Rust 1.62.0

https://github.com/rust-lang/rust/pull/94927

If approved, the following will be possible in stable compiler versions.

fn main() {
    let outer_opt = Some(Some(1i32));
    
    if let Some(inner_opt) = outer_opt
        && let Some(number) = inner_opt
        && number == 1
    {
        println!("Peek a boo");
    }
}