Derive for KDL

[tailhook]

Hi! I think it's worth to make a derive macro for decoding and validating KDL. This is a bit different from #17, as that has some specific mapping between KDL and serde structure. And I'm proposing something, more like in clap.

So for this example from help (adjusted a bit):

contents {
  section "First section" style="emphasized" {
    paragraph "This is the first paragraph"
    paragraph "This is the second paragraph"
  }
}

You could have structures like:

#[derive(kdl::Decode)]
struct Contents {
    #[kdl(children, filter(node_name="section"))]
    sections: Vec<Section>
}

#[derive(kdl::Decode)]
struct Section {
    #[kdl(argument)]
    title: String,
    #[kdl(property, default="normal")]
    style: String,
    #[kdl(children)]
    children: Vec<Block>
}

#[derive(kdl::Decode)]
enum Block {
    #[kdl(node_name="paragraph")]
    Paragraph(String),
    #[kdl(node_name="figure")]
    Figure(Figure),
}

And this could serve as both: more convenient structure to work with data and to validate that there are no extra arguments and properties (unless there is an explicit extension point) and types of all arguments and properties are correct.

What do you think?

[tailhook]

Implemented in https://docs.rs/knuffel/ (although, that also uses its own parser, so not directly related to this project)