Add an Option to use a Format String to Structs

[Roland Fredenhagen]

My Idea is to use something similar to thiserror where you specify a format string that get's filled with the fields:

#[derive(Error, Debug)]
pub enum DocumentError {
    #[error("Invalid {0}: `{1}`")]
    InvalidAttribute(String, String),
}

This could look like this for Debug of structs:

#[derive(Derivative)]
#[derivative(Debug(format="({x},{y}: {w})"))]
pub struct Point {
    pub x: f32,
    pub y: f32,
    pub w: f32,
}

I think most of the Time this is enough for formatting, and you don't actually need a function for that.