stream: Break up the onread function

A primary motivation of this is to make the onread function more
inline-friendly, but also to make it more easy to explore not having
onread at all, in favor of always using push() to signal the end of
reading.
stream: Add stream.unshift(chunk)
stream: There is no _read cb, there is only push

This makes it so that `stream.push(chunk)` is the only way to signal the
end of reading, removing the confusing disparity between the
callback-style _read method, and the fact that most real-world streams
do not have a 1:1 corollation between the "please give me data" event,
and the actual arrival of a chunk of data.

It is still possible, of course, to implement a `CallbackReadable` on
top of this.  Simply provide a method like this as the callback:

    function readCallback(er, chunk) {
      if (er)
        stream.emit('error', er);
      else
        stream.push(chunk);
    }

However, *only* fs streams actually would behave in this way, so it
makes not a lot of sense to make TCP, TLS, HTTP, and all the rest have
to bend into this uncomfortable paradigm.
doc: Provide 2 examples of SimpleProtocol parser

The first example uses Readable, and shows the use of
readable.unshift().  The second uses the Transform class, showing that
it's much simpler in this case.