serde 0.8.21

Implement skip_serializing for enum variants (#653, thanks @shinglyu)
```
#[derive(Serialize)]
enum E {
    A(bool),
    B(String),
    #[serde(skip_serializing)]
    C(NotSerializable),
}
```
Serialization will fail with a message if that particular variant is the one being serialized.

"The enum variant E::C cannot be serialized"