Add a Deserializer

[dtolnay]

use anyhow::Result;
use serde::Deserialize;
use serde_yaml::Value;

fn main() -> Result<()> {
    let input = "---\nk: 107\n";
    let de = serde_yaml::Deserializer::from_str(input);
    let value = Value::deserialize(de)?;
    println!("{:?}", value);
    Ok(())
}

Closes #44.