Parse rhs of select! arms using match-arm rules

[dtolnay]

Fixes #2824.

This is a breaking change because, for example, the following program would break.

use futures::{future, select};

fn main() {
    let _ = async {
        select! {
            () = future::ready(()) => {}()
        }
    };
}

[taiki-e]

Thanks!