feat(DeriveAttribute) Implement derive macro attribute for format

[johnhurt]

Build on the newly-introduced derive-attribute framework to allow fields in structs to be written to the final value as a formatted string instead of their actual value. This allows for types that don't (and can't or won't) implement Valuable to be included in the value

[taiki-e]

Thanks for the PR!

• This appears to bypass Is it possible to allow to visit a value referencing data owned by the current function? #43 well, which is interesting.
• This currently requires allocation, which is unfortunate given that there are APIs such as format_args that do not require allocation.

[johnhurt]

Ooooooh, I see what you mean. Let me tackle those one at a time

1. That doesn't seem like a good thing. I didn't think about the ability to leak things out with a format. I will change this from #[valuable(format = "{}")] to just #[valuable(debug)] to avoid
2. There doesn't seem to be a way around allocating to do what I'm trying to do. Even to complete Consider other Value primitive variants #4 I think would require allocation :-/ ... Would you be open to the idea of adding an opt-in alloc feature that would gate the #[valuable(debug)] attribute?

[taiki-e]

Adding Debug/Display (or Debugable/Displayable mentioned in #43 (comment)) variant to valuable::Value and then adding #[valuable(debug)]/#[valuable(display)] that pass the field as Value::Debug/Value::Display to derive seem to good idea to me.

[johnhurt]

I'm closing this in favor of #133