Keep state of the Rust Playgrounds when navigating between slides

Today, the embedded Playgrounds are reset when you navigate between slides. This has caused problems: if people navigate away from a slide to look something up, they've suddenly lost their work.

This is actually the reason why we don't make the exercises editable directly in the slides: too many people have lost work because of this.

We should be able to store the current state in the browser's local storage.