Update TODO for short header versions:

< 	o Reorder physical storage order to reduce padding?
<
< 	  This involves having the user-specified order of columns
< 	  be different from the physical order.  SELECT * would
< 	  need to reorder the physical values to match the
< 	  user-specified ordering.
<
< 	o Store disk pages with no alignment/padding?
<
< 	  This necessitates adding CPU-required padding when moving
< 	  rows from disk to memory.
<
< 	  One idea is to store the header in network byte order (high bits
< 	  first), and read the high bits to determine the header length.
< 	  http://archives.postgresql.org/pgsql-hackers/2006-09/msg00848.php
> 	  One idea is to create zero-or-one-byte-header versions
> 	  of varlena data types.  In involves setting the high-bit and
> 	  0-127 length in the single-byte header, or clear the high bit
> 	  and store the 7-bit ASCII value in the rest of the byte.
> 	  The small-header versions have no alignment requirements.
> 	  http://archives.postgresql.org/pgsql-hackers/2006-09/msg01372.php