Tidy up: make Option and Vec pass through TextFragment.

Tidy up: make Option and Vec pass through TextFragment.

Now if you have an Option<TextFragment>, or a Vec<TextFragment> (or
even, heaven help you, an Option<Vec> or a Vec<Option<Vec>> etc), you
can just call .render() on the whole thing without having to manually
loop over it. I'm starting to like this feature of Rust!