This generalizes the `New` mailbox to save the N latest elements
The `New` mailbox is now explained as a special case of the `Newest`
mailbox and the tutorial has been updated to explain how this mailbox
The previous instance did the wrong thing: if one 'Input' terminated the whole
'Alternative' terminated. The current instance ensures that both have to
terminate before the combined 'Input' terminates.
Thanks to @boothead for catching this bug.