Add Option<DateTime> outputs for various serde helpers (ts_seconds, etc)

[manifest]

The following doesn't work now:

#[derive(Debug, Serialize, Deserialize)]
struct Object {
    #[serde(with = "ts_seconds")]
    time: Option<DateTime<Utc>>,
}

error[E0308]: mismatched types
  --> src/db/rtc.rs:36:17
   |
36 | #[derive(Debug, Serialize, Deserialize)]
   |                 ^^^^^^^^^ expected struct `chrono::datetime::DateTime`, found enum `std::option::Option`
   |
   = note: expected type `&chrono::datetime::DateTime<chrono::offset::utc::Utc>`
              found type `&'__a std::option::Option<chrono::datetime::DateTime<chrono::offset::utc::Utc>>`

error[E0308]: match arms have incompatible types
  --> src/db/rtc.rs:36:28
   |
36 | #[derive(Debug, Serialize, Deserialize)]
   |                            -----------
   |                            |
   |                            expected enum `std::option::Option`, found struct `chrono::datetime::DateTime`
   |                            match arm with an incompatible type
   |                            in this macro invocation
   |
   = note: expected type `std::option::Option<chrono::datetime::DateTime<_>>`
              found type `chrono::datetime::DateTime<_>`

error[E0308]: match arms have incompatible types
  --> src/db/rtc.rs:36:28
   |
36 | #[derive(Debug, Serialize, Deserialize)]
   |                            -----------
   |                            |
   |                            expected enum `std::option::Option`, found struct `chrono::datetime::DateTime`
   |                            match arm with an incompatible type
   |                            in this macro invocation
   |
   = note: expected type `std::option::Option<chrono::datetime::DateTime<_>>`
              found type `chrono::datetime::DateTime<_>`

[manifest]

I've just added a PR with these new modules:

• ts_nanoseconds_option
• ts_milliseconds_option
• ts_seconds_option

They all reuse current Serializers and Deserializers' Visitors.

[elpiel]

Is this still on the roadmap/workings?

[quodlibetor]

This is now merged.