Rust: Support tagged unions

[phrohdoh]

Currently quicktype always emits serde(untagged) for unions.

Serde supports multiple tagged union forms:

• externally
• internally
• adjacently

Externally

#[derive(Serialize, Deserialize)]
enum Message {
    Request { id: String, method: String, params: Params },
    Response { id: String, result: Value },
}

{"Request": {"id": "...", "method": "...", "params": {...}}}

Internally

#[derive(Serialize, Deserialize)]
#[serde(tag = "type")]
enum Message {
    Request { id: String, method: String, params: Params },
    Response { id: String, result: Value },
}

{"type": "Request", "id": "...", "method": "...", "params": {...}}

Adjacently

#[derive(Serialize, Deserialize)]
#[serde(tag = "t", content = "c")]
enum Block {
    Para(Vec<Inline>),
    Str(String),
}

{"t": "Para", "c": [{...}, {...}]}
{"t": "Str", "c": "the string"}

[schani]

This requires a bit more work to allow more generic unions, at least for the more interesting cases. See #493 and #257

[schani]

The one that's most interesting to me is the internal variant. It's also what we'd need for GraphQL.

Adjacent just seems like a special case of that, in terms of how the JSON is encoded.

And external could be represented in quicktype's IR as a union like

  { "Request": { "id": ... } }
| { "Response": { "id": ... } }