Add an extra header byte to TOAST-pointer datums to represent their size

Add an extra header byte to TOAST-pointer datums to represent their size
explicitly.  This means a TOAST pointer takes 18 bytes instead of 17 --- still
smaller than in 8.2 --- which seems a good tradeoff to ensure we won't have
painted ourselves into a corner if we want to support multiple types of TOAST
pointer later on.  Per discussion with Greg Stark.