Support deserializing from &RawValue

[dtolnay]

use std::collections::BTreeMap;

fn main() {
    let j = r#" {"x": {"k": "v"}} "#;
    let deferred: BTreeMap<&str, &serde_json::value::RawValue> = serde_json::from_str(j).unwrap();
    let x: BTreeMap<&str, &str> = serde::Deserialize::deserialize(deferred["x"]).unwrap();
    println!("{:?}", x);
}