Define `empty` for `Filler`?

[yanganto]

The #[filler(empty_value=value)] is implemented. However, we do not need so much macros. The filler rely on is_empty function. The user can implement the is_empty for his customized struct. We will consider add #[filler(empty=fn)]

#[derive(struct_patch::Filler, Debug, PartialEq, Default)]
struct Color {
    R: u8,
    G: u8,
    B: u8,
    #[filler(empty_value=0)]
    // or #[filler(default_is_empty)]
    alpha: u8,
}

[yanganto]

The #[filler(full=fn)], #[filler(full_value=value)], and #[filler(default_is_empty)] will not implement. Because user can add filler to gather before applying so the fulls are not useful, and the #[filler(default_is_empty)] can be replaced by #[filler(empty=default)].