Add the full set of comparison functions for type TID, including a btree

Add the full set of comparison functions for type TID, including a btree
opclass.  This is not so much because anyone's likely to create an index
on TID, as that sorting TIDs can be useful.  Also added max and min
aggregates while at it, so that one can investigate the clusteredness of
a table with queries like SELECT min(ctid), max(ctid) FROM tab WHERE ...
Greg Stark and Tom Lane