Optionally serialize options

[nikclayton-dfinity]

[Posting as new issue because I think the comment on the other issue may have been missed]

I don't think this is quite right.

Consider the following:

#[derive(Clone, Debug, Default, Serialize, KV, SerdeValue)]
struct A {
    #[serde(skip_serializing_if = "Option::is_none")]
    b: Option<B>
}

#[derive(Clone, Debug, Default, Serialize, SerdeValue)]
struct B {
    #[serde(skip_serializing_if = "Option::is_none")]
    first: Option<String>,
    second: Option<String>
}

[...]

let a = A::default();

With (at least) slog_json, if I try and log that (info!(logger, "msg"; a);) I get the output

{
  "b": null
}

I would expect {} as the output.

If I partially construct B like this:

let a = A {
    b: Some(B {
        first: Some("hello world".into()),
        second: None,
    }),
};

and log a again I get:

{
   "b": {
     "first": "hello world"
   }
}

So skipping the option on the second field has worked.

If I remove the KV from the derive line, and write the log like info!(logger, "msg"; "a" => a); then the optional fields are correctly skipped, but everything is now under the "a" key in the output.

Originally posted by @nikclayton-dfinity in #5 (comment)