Should it be `fmt::Write` instead of `io::Write`?

fmt::Write makes the assumption that the inputs will be unicode, which is usually a good assumption for textual formats, and I notice that assumption is already being assumed in Element::to_string, which panics if the serialization of the tree isn't unicode.

fmt::Write also doesn't do flushing. Are you using flushing?

I ask because outputting only to an io::Write makes it annoying to use Elements into format functions, they can't implement Display. I don't actually know how to make an adaptor between the two writes, so I'm just writing to a string then fmting the string.

Start by reviewing the Element::to_string entry point and the current serialization output API. Determine whether fmt::Write can replace io::Write, whether flushing is required, and how Display integration should work; done means a decided API direction with corresponding behavior and tests identified.