Support using DeserializeSeed with StreamDeserializer

[dtolnay]

Currently the stream API looks like:

impl<'de, R, T> Iterator for StreamDeserializer<'de, R, T>
    where R: Read<'de>,
          T: Deserialize<'de>,
{
    type Item = Result<T>;

    fn next(&mut self) -> Option<Result<T>>;
}

There should be some way to pass in a DeserializeSeed to get the next value.

[spring4175]

I've found a need for this feature. I'd be happy to take an attempt at a PR if you have a general design in mind.

[MiSawa]

FWIW, serde_yaml supports this by returning Iterator<Item=Self> on Deserializer::into_iter(), so we can do

for de in serde_yaml::Deserializer::from_reader(std::io::stdin()).into_iter() {
    let seed = todo!(); // Construct whatever DeserializeSeed
    let _value = seed.deserialize(de)?;
}

[conradludgate]

I'm planning to implement a patch for this. My current design is to provide a

fn next_seed<S>(&mut self, seed: S) -> Option<Result<T, Error>>
where
    S: DeserializeSeed<'de, Value = T>;

T no longer needs to be Deserialize to use this method.