Extend the public interface with Deserializer

[xamgore]

If there was an error while matching the parsed data with the defined struct, the path a.b.c[0] may be returned. One could use serde_path_to_error crate for that, but it requires access to the Deserializer. This PR is for that.

    #[derive(Deserialize)]
    struct Data { a: String };

    let j = r#"{ b: 42 }"#;

    // Some Deserializer.
    let jd = &mut serde_json5::Deserializer::from_str(j)?;

    let result: Result<Package, _> = serde_path_to_error::deserialize(jd);
    match result {
        Ok(_) => panic!("expected a type error"),
        Err(err) => {
            let path = err.path().to_string();
            println!("{}", path);
        }
    }

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[xamgore]

Signed CLA.

[gebressler]

LGTM, sorry for the long wait!