Stack overflow for very long words

If I add a very long word to the index (8000 characters is enough on my machine), I get a stack overflow while serialising it.

Here's how I was testing:

````
fn main() {
    let mut index = Index::new(&["body"]);
    let long_string = "a".repeat(8000);
    index.add_doc("1", &[long_string]);
    index.to_json_pretty();
}
````
