Support flatten within struct variant

[dtolnay]

#[derive(Serialize, Deserialize)]
enum Foo {
    A {
        #[serde(flatten)]
        fields: HashMap<String, String>,
    }
}

error: proc-macro derive panicked
 --> src/main.rs:6:21
  |
6 | #[derive(Serialize, Deserialize)]
  |                     ^^^^^^^^^^^
  |
  = help: message: #[serde(flatten)] cannot be used within enums

[dtolnay]

@jplatte @tomjakubowski ready to go in Serde 1.0.47!