Re: [streams] Move BYOBRequest after BYOBController and add FS example (b430f33)

BTW, the example below also need to handle `bytesRead` as well as the issue @yutakahirano pointed out at https://github.com/whatwg/streams/pull/430#issuecomment-192093149 in the newly added example.

`buffer` can be filled partially (even if the API blocks until CHUNK_SIZE bytes are filled, we may eventually need to return a partially filled buffer at the end of the file), and there's no way to know that for the user of the stream.

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