Allow for serde_json::Value in body/return types

[ahl]

Currently we fail on an unreachable!(): not quite true--obviously--since I forgot about permissive types such as serde_json::Value. This fix adds handling for that type. It also improves the panic message for the empty set type.

Note that I ran the resulting schema through openapi-generator and the model looks exactly as we would hope:

pub struct BodyParam {
    #[serde(rename = "_any")]
    pub _any: Option<serde_json::Value>,
    #[serde(rename = "_x")]
    pub _x: String,
}

(albeit with some extraneous renames... generated code, what can you do)

[davepacheco]

LGTM -- fixes the bug I was seeing.

[davepacheco]

Seems like we can probably skip a change log entry for this, too.

[ahl]

We could add: the use of permissive schemas (e.g. serde_json::Value) in API types is allowed