Implemented in #57375

This will make working with durations more ergonomic. Compare:

// Convenient, but deprecated function.
thread::sleep_ms(2000);

// The current canonical way to sleep for two seconds.
thread::sleep(Duration::from_secs(2));

// Sleeping using one of the new constants.
thread::sleep(2 * SECOND);

impl Duration {
    pub const SECOND: Duration = Duration::from_secs(1);
    pub const MILLISECOND: Duration = Duration::from_millis(1);
    pub const MICROSECOND: Duration = Duration::from_micros(1);
    pub const NANOSECOND: Duration = Duration::from_nanos(1);
    pub const MAX: Duration = ...;
}