Add a trait TextFragmentOneLine.

Add a trait TextFragmentOneLine.

This is a slight refinement of ordinary TextFragment, to be
implemented by types that render to exactly one line of text always,
as opposed to being able to render to 0 or to more than one. The idea
is that if a type implements this trait, then the compiler can _know_
there's one line of text coming out of its render method, instead of
having to call .iter().exactly_one().unwrap() and think hard about
whether I'm risking a runtime failure.

I could implement this trait for a lot more things in text.rs, but for
the moment, I'm only bothering to put it on the one that I need it for
right now.