Add #[default] macro (or different name)

[AldaronLau]

An attribute macro to create a separate trait that can set default values for associated items, and default blocks for methods and associated functions. Everything that implements the "default" trait will implement the trait.

use traitful::default;

#[default(
    TraitDefault => Default::default(),
    TraitUnimplemented => unimplemented!(),
)]
pub trait Trait {
    fn uwu() -> i32;
}

// becomes:

pub trait Trait {
    fn uwu() -> i32;
}

pub trait TraitDefault {
    fn uwu() -> i32 {
        Default::default()
    }
}

pub trait TraitUnimplemented {
    fn uwu() -> i32 {
        unimplemented!()
    }
}

mod traitful_default__ {
    use super::*;

    impl<T: TraitDefault> Trait for T {
        fn uwu() -> i32 {
            <Self as TraitDefault>::uwu()
        }
    }

    impl<T: TraitUnimplemented> TraitDefault for T {
        fn uwu() -> i32 {
            <Self as TraitUnimplemented>::uwu()
        }
    }
}