Use default literals instead of Default::default() in const initialization

[lucic71]

Default::default() is not a const function, hence it cannot be used to initialize global objects such as pub static mut static_outer: Outer = Outer::default().

To overcome this limitation, emit the default literal for the object, for example:

pub static mut static_outer: Outer = Outer {
    p1: std::ptr::null_mut(),
    p2: std::ptr::null(),
    arr: [std::ptr::null_mut(); 3],
    cp: std::ptr::null(),
    pp: std::ptr::null_mut(),
    inner: Inner {
        v: 0_i32,
        name: std::ptr::null(),
    },
    x: 0_i32,
    fn_: None,
};