Support serde(bound = "...") attribute on enum variants

[dtolnay]

Variants support with so it makes sense to support bound as well. The two commonly go together.

#[derive(Serialize, Deserialize)]
enum Repro<T> {
    #[serde(with = "v")]
    #[serde(bound(serialize = "T: Serialize", deserialize = "T: Deserialize<'de>"))]
    V(T),
}

Currently this fails with:

 |
 | #[derive(Serialize, Deserialize)]
 |                     ^^^^^^^^^^^
 |
 = help: message: unknown serde variant attribute `bound`

[dtolnay]

Fixed in be7fe2a.