1-byte packed varlena headers

Updated patch at:

 http://community.enterprisedb.com/varlena/patch-varvarlena-12.patch.gz

This fixes a rather critical oversight which caused it all to appear to work
but not actually save any space. I've added an assertion check that the
predicted tuple size matches the tuple size generated by heap_form*tuple.

--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com