Dodge a macro-name conflict with Perl.

Some versions of Perl export a macro named HS_KEY.  This creates a
conflict in contrib/hstore_plperl against hstore's macro of the same
name.  The most future-proof solution seems to be to rename our macro;
I chose HSTORE_KEY.  For consistency, rename HS_VAL and related macros
similarly.

Back-patch to 9.5.  contrib/hstore_plperl doesn't exist before that
so there is no need to worry about the conflict in older releases.

Per reports from Marco Atzeri and Mike Blackwell.