Implement Sum type for duration?

[jakevossen5]

Here is a basic example:

use std::vec;

use chrono::{DateTime, Duration, NaiveDateTime, TimeZone, Utc};
fn main() {
    let dt = Utc.ymd(1970, 1, 1).and_hms_nano(0, 0, 1, 444);
    let dt_2 = Utc.ymd(1977, 1, 1).and_hms_nano(0, 0, 1, 444);
    let dt_3 = Utc.ymd(1970, 1, 1).and_hms_nano(0, 0, 1, 444);
    let dt_4 = Utc.ymd(2020, 1, 1).and_hms_nano(0, 0, 1, 444);

    let dur_1 = dt_2 - dt;
    let dur_2 = dt_3 - dt;
    let dur_3 = dt_4 - dt;

    let durations = vec![dur_1, dur_2, dur_3];

    let total: Duration = durations.iter().sum();
}

Running cargo check results in

error[E0277]: the trait bound `chrono::Duration: Sum<&chrono::Duration>` is not satisfied
  --> src/main.rs:17:44
   |
17 |     let total: Duration = durations.iter().sum();
   |                                            ^^^ the trait `Sum<&chrono::Duration>` is not implemented for `chrono::Duration`

The Duration in the std has Sum implemented (https://doc.rust-lang.org/std/time/struct.Duration.html#impl-Sum%3CDuration%3E), it would be nice if chrono had it too. If this is a feature that should be added, I would be willing to take a look at implementing it.

[quodlibetor]

No reason it isn't implemented, a PR would be welcome!