Use a different representation for Duration

[brson]

The Duration type introduced in #15934 looks like

pub struct Duration {                 
    days: i32,                                                                
    secs: u32,  // Always < SECS_PER_DAY                                     
    nanos: u32, // Always < NANOS_PR_SECOND                                                   
}

Per @kballard's [comment] this could be

pub struct Duration {                                                                 
    secs: i64,       
    nanos: u32, // Always < NANOS_PR_SECOND                                                   
}

[ruuda]

I'd like to work on this.