Expose serde_json::de::Read to use in where clauses

[dtolnay]

0.8.*

pub struct Client {
    reader: StreamDeserializer<serde_json::Value, Bytes<TcpStream>>,
}

fn f(c: Client) { /* ... */ }

0.9.0

pub struct Client<D> {
    reader: D,
}

fn f<D>(c: Client<D>) where D: Iterator<Item = Result<serde_json::Value, serde_json::Error>> { /* ... */ }

now

pub struct Client<R> {
    reader: StreamDeserializer<R, serde_json::Value>,
}

fn f<R>(c: Client<R>) where R: serde_json::de::Read { /* ... */ }

Fixes #234. cc @sorccu