Add #[impl_default]

[dhardy]

This does not use valid Rust syntax for field initialisers, nevertheless it works within impl_scope:

impl_scope! {
    #[impl_default]
    struct Person {
        name: String = "Jane Doe".to_string(),
        age: u32 = 72,
        occupation: String,
    }
}

TODO: some tidy up of this PR before merging.