default! macro for Rust

Sometimes you want to create nested structures, set values for some the fields and use default value for most of the other, for example:

```

[derive(Default)]

struct Window { title: &str, border: Border, /* 10 other fields */ }

[derive(Default)]

struct Border { width: f64, /* 5 other fields*/ }

fn foo() { let w1 = Window { title: "Test", border: Border { width: 10.0, ..Border::Default}, ..Window::default()};

// with the macros:
let w2 = default!( Window { title: "Test", border: Border {  width: 10.0} });

} ```