serde_json 0.9.8 on Rust Cargo

Implement serde::Deserializer for &serde_json::Value (#261)

This can be convenient for deserializing from indexed content:

let v = json!({ "m": { "serde": 1, "json": 100 } });

let m: BTreeMap<String, u8> = Deserialize::deserialize(&v["m"])?;

Add an entry API to the serde_json::Map type, just like what BTreeMap has (#236)

let mut map = serde_json::Map::new();

map.entry("serde").or_insert(json!(12));

match map.entry("serde") {
    Entry::Vacant(vacant) => {
        // can insert
    }
    Entry::Occupied(occupied) => {
        // can read, mutate, remove
    }
}

assert_eq!(map["serde"], 12);

Quit using voldemort types to parameterize the deserializer; this should make storing serde_json::Deserializer and StreamDeserializer in structs more convenient (#260)