Warn about overlapping match arm

[antoyo]

Hello.
It would be nice to have a warning for the following code:

let x = 5;
match x {
    1 ... 10 => println!("1 ... 10"),
    5 ... 15 => println!("5 ... 15"),
    _ => (),
}

because the first two match arms are overlapping.

Thanks.