Add a spec for an extended API for binary data handling

Here and elsewhere, technically the ES spec does not use `.` for accessing internal slots: only for accessing records. See https://github.com/domenic/promises-unwrapping/issues/49 and linked thread.

However, this is annoying enough that I have willfully violated it so far, and might continue to do so, or might consider a different syntax like "_stream_→[[state]]" or similar. Just letting you know.