Add proc-macros for DeepClone and ByteRepr

[lucic71]

Currently we generate:

#[derive(Default)]
pub struct woff2_Point {
    pub x: Value<i32>,
    pub y: Value<i32>,
    pub on_curve: Value<bool>,
}
impl Clone for woff2_Point {
    fn clone(&self) -> Self {
        let mut this = Self {
            x: Rc::new(RefCell::new((*self.x.borrow()))),
            y: Rc::new(RefCell::new((*self.y.borrow()))),
            on_curve: Rc::new(RefCell::new((*self.on_curve.borrow()))),
        };
        this
    }
}
impl ByteRepr for woff2_Point {
    fn to_bytes(&self, buf: &mut [u8]) {
        (*self.x.borrow()).to_bytes(&mut buf[0..4]);
        (*self.y.borrow()).to_bytes(&mut buf[4..8]);
        (*self.on_curve.borrow()).to_bytes(&mut buf[8..9]);
    }
    fn from_bytes(buf: &[u8]) -> Self {
        Self {
            x: Rc::new(RefCell::new(<i32>::from_bytes(&buf[0..4]))),
            y: Rc::new(RefCell::new(<i32>::from_bytes(&buf[4..8]))),
            on_curve: Rc::new(RefCell::new(<bool>::from_bytes(&buf[8..9]))),
        }
    }
}

That's a lot of code. We can write 2 proc-macros: DeepClone and ByteRepr and generate the following code instead:

#[derive(Default, DeepClone, ByteRepr)]
pub struct woff2_Point {
    pub x: Value<i32>,
    pub y: Value<i32>,
    pub on_curve: Value<bool>,
}

[lucic71]

I think we can apply the same strategy for accessors of unions and our custom Default implementation for refcount structs.