Utf8 support with stringview

Closed #19 in favor of this, since we really need to ship with StringView for default as a demonstration of working UTF-8, even if people don't use it in practice.

We probably also need to add more documentation, so it's obvious to noobs (in this case, including me) how Unicode works in JavaScript, and how it interacts with Weblua, since that stuff isn't terribly intuitive.