Add streaming deserializer mode

[tinou98]

This allows to efficiently de-serialize array ([ val, val ]), map, ({ key: val, key: val }), and list (val val) of any type.

It follows some idea discussed in #526 (comment)

Full manual example:

    let deserializer = serde_json::Deserializer::from_str(
        r#"[1, 5, {"a": 1, "b": 2, "c": [1, 2], "d": 3}, 0]
        {
            "a"
            :
            { "b" : 1 },
            "d": 5
        }
        "#,
    );

    let stream = Stream::new(deserializer);

    let mut stream = stream.enter_array().unwrap();
    stream.next_value::<usize>().unwrap(); // 1
    stream.next_value::<usize>().unwrap(); // 5

    let mut stream = stream.enter_map().unwrap();
    stream.next_value::<usize>().unwrap(); // a, 1
    stream.next_value::<usize>().unwrap(); // b, 2
    let (key, mut stream) = stream.enter_array().unwrap(); // key == c
    stream.next_value::<usize>().unwrap(); // 1
    stream.next_value::<usize>().unwrap(); // 2

    let mut stream = stream.end_array().unwrap();
    stream.next_value::<usize>().unwrap(); // d, 3
    let mut stream = stream.end_map().unwrap();

    stream.next_value::<usize>().unwrap(); // 0
    let stream = stream.end_array().unwrap();

    let stream = stream.enter_map().unwrap();

    let (key, mut stream) = stream.enter_map().unwrap(); // key == a
    stream.next_value::<usize>().unwrap(); // b, 1
    let mut stream = stream.end_map().unwrap();

    stream.next_value::<usize>().unwrap(); // d, 5
    let stream = stream.end_map().unwrap();

    stream.end().unwrap();

Iterator mode :

    let deserializer = serde_json::Deserializer::from_str(r#"[1, 2, 5, 6, 0]5"#);

    let stream = Stream::new(deserializer);

    let mut stream = stream.enter_array().unwrap();
    let iter = stream.iter::<usize>();
    for res in iter {
        let el = res.unwrap();
        // ...
    }

    let mut stream = stream.end_array().unwrap();
    stream.next_value::<usize>().unwrap(); // 5
    stream.end().unwrap();

[dtolnay]

Thanks, this looks promising.

I am closing the PR because like in #526 I'd like for this to be pursued and released in an external crate at first. I would only consider accepting an implementation in serde_json once there is sufficient usage experience with that.

[tinou98]

I tried to move this to separate crate, but I'm facing a big problem : I cannot use serde_json::Deserializer. If I want to use it, I need to give it the Read object and there is no way to get it back.

Would it be possible to add Read implementation to mutable reference to Read ?

impl<'de, R: Read<'de>> Read<'de> for &mut R;

Because of the Sealed trait, we can't implement Read in other create.