remove a todo

[yshavit]

I looked into it, and replacing the callback for without_wrapping with a PreWriter (or even just a W) wouldn't be worth it. I'd still have to borrow against self.words_buffer, unless I wanted an even more complicated thing where the writes happen and then report back to the WordsBuffer how many chars go written without wrapping. That's certainly possible, but more complicated than I think is worthwhile.

Instead, I added #[must_use] to the return of set_word_boundary. That gives me decent (if not perfect) protection against forgetting to restore it, and keeps things simple.

While I was here, I cleaned up a bunch of <W> ... where W:'s, and turned them into impl.