Struct DateTimeCopy item path

use chrono::prelude::*;
let chrono_dt: chrono::DateTime<Utc> = "2014-11-28T12:00:09Z".parse()?;
let bson_dt: bson::DateTime = chrono_dt.into();
let bson_dt = bson::DateTime::from_chrono(chrono_dt);
let back_to_chrono: chrono::DateTime<Utc> = bson_dt.into();
let back_to_chrono = bson_dt.to_chrono();

let dt = bson::DateTime::builder().year(1998).month(2).day(12).minute(1).millisecond(23).build()?;
let expected = bson::DateTime::parse_rfc3339_str("1998-02-12T00:01:00.023Z")?;
assert_eq!(dt, expected);

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
struct Foo {
    // serializes as a BSON datetime.
    date_time: bson::DateTime,

    // serializes as an RFC 3339 / ISO-8601 string.
    chrono_datetime: chrono::DateTime<chrono::Utc>,
}

let f = Foo { date_time: bson::DateTime::now(), chrono_datetime: chrono::Utc::now() };
println!("{:?}", bson::to_document(&f)?);

{ "date_time": DateTime("2023-01-23 20:11:47.316 +00:00:00"), "chrono_datetime": "2023-01-23T20:11:47.316114543Z" }

let f = Foo { date_time: bson::DateTime::now(), chrono_datetime: chrono::Utc::now() };
println!("{}", serde_json::to_string(&f)?);

{"date_time":{"$date":{"$numberLong":"1674504029491"}},"chrono_datetime":"2023-01-23T20:00:29.491791120Z"}

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
struct Foo {
    // serializes as a BSON datetime.
    date_time: bson::DateTime,

    // serializes as an RFC 3339 / ISO-8601 string.
    chrono_datetime: chrono::DateTime<chrono::Utc>,

    // serializes as a BSON datetime.
    // this requires the "chrono-0_4" feature flag
    #[serde(with = "bson::serde_helpers::chrono_datetime_as_bson_datetime")]
    chrono_as_bson: chrono::DateTime<chrono::Utc>,

    // serializes as an RFC 3339 / ISO-8601 string.
    #[serde(with = "bson::serde_helpers::bson_datetime_as_rfc3339_string")]
    bson_as_string: bson::DateTime,
}

use serde::{Deserialize, Serialize};
use bson::doc;

#[serde_with_3::serde_as]
#[derive(Deserialize, Serialize, PartialEq, Debug)]
struct Foo {
  /// Serializes as a BSON datetime rather than using [`chrono::DateTime`]'s serialization
  #[serde_as(as = "Option<bson::DateTime>")]
  as_bson: Option<chrono::DateTime<chrono::Utc>>,
}

let dt = chrono::Utc::now();
let foo = Foo {
  as_bson: Some(dt),
};

let expected = doc! {
  "as_bson": bson::DateTime::from_chrono(dt),
};

assert_eq!(bson::to_document(&foo)?, expected);