Assorted improvements in contrib/hstore.

Assorted improvements in contrib/hstore.

Remove the 64K limit on the lengths of keys and values within an hstore.
(This changes the on-disk format, but the old format can still be read.)
Add support for btree/hash opclasses for hstore --- this is not so much
for actual indexing purposes as to allow use of GROUP BY, DISTINCT, etc.
Add various other new functions and operators.

Andrew Gierth