Rust SmartDefault

Custom derive for automatically implementing the Default trait with customized default values:

```rust

[derive(SmartDefault)]

enum Foo { Bar, #[default] Baz { #[default = "12"] a: i32, b: i32, #[default = r#""hello""#] c: &'static str, }, Qux(i32), }

assert!(Foo::default() == Foo::Baz { a: 12, b: 0, c: "hello" }); ```