Since we're always using `ReadableStream`, we should just get rid of `ResultPiper`.
This also lets us replace things like `bufferedReadFromReadableStream` with a `TransformStream` that does the same thing, so that it's `TransformStream`s all the way down.
Finally, we can get rid of the one-off call to `renderToReadableStream` and just use `renderToStream` whenever we're rendering a concurrent tree.
