derive Render

[killercup]

Offer a simple derive that used some default style and allows to do

#[derive(Serialize, RenderOutput)]
struct Foo { name: String, code: i32}

out.print(Foo { name: "hallo".into(), code: 42 })?;

The goal is to let people write even less code. This is an experiment, though, and only in addition to "manually" implemementing using the macros.

1. Create proc macro crate output_derive
2. Add a simple derive macro RenderOutput

• only works for structs for now
• generates the most trivial impl Render for $type
  • human output: just list all the fields separated by newlines
  • json output: same as render_json!();

3. Add attributes to fields to add some formatting/converting.

[killercup]

Works, and the fuzzy requirement of "formatting attributes" should be tracked in another issue.