fn format_time(duration: &Duration) -> f64 {
        if duration.num_milliseconds() == 0 {
            if let Some(v) = duration.num_microseconds() {
                (v as f64) / 1000.0
            } else {
                0.0
            }
        } else {
            duration.num_milliseconds() as f64
        }
    }
Formats a duration for printing the statistics, in terms of milliseconds and microseconds.
weld/src/util/stats.rs