Method for URL-safe project serialization

It's not as easy as you'd think to serialize the state of a project for e.g. sticking in a URL, because `btoa` throws on non-Latin-1 characters, so special handling there is required. Also, base64url is a better scheme (`-` and `_` instead of `+` and `/`), because it's less likely to get mis-encoded somehow (especially the `+` which will become a space).

Let's add something like `serialize()` and `deserialize()` to `project` and `ide`, to make this use case simpler.