Move IPython.core.payloadpage to this package ?

```
ipykernel/ipykernel/zmqshell.py
 23:from IPython.core import page, payloadpage
547:        self.set_hook("show_in_pager", page.as_hook(payloadpage.page), 99)
```

Seem to be the only thing that refer to IPython.core.payloadpage submodule, even in IPython itself; 
so I think it should be moved here.  