Add a thing to make using BufferCollator as some sort of queue easy and ...

...fast
untested :)

  def test_max_bytes(self, buf):
      buf.add_bytes(b"abc")
      view = buf.view()
      collator = BufferCollator()
      collator.append(view)
      collator.append(view)
      view = collator.collapse(max_bytes=2)
      assert view == b"ab"
      next_view = collator.collapse(max_bytes=4)
      assert next_view == b"cabc"