Support functions for index opclasses should be immutable.

Found by running opr_sanity on contrib modules.

contrib/hstore/hstore.sql.in

@@ -186,37 +186,37 @@ CREATE TYPE ghstore (
 CREATE FUNCTION ghstore_compress(internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE FUNCTION ghstore_decompress(internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE FUNCTION ghstore_penalty(internal,internal,internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C IMMUTABLE STRICT;
 
 CREATE FUNCTION ghstore_picksplit(internal, internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE FUNCTION ghstore_union(internal, internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE FUNCTION ghstore_same(internal, internal, internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE FUNCTION ghstore_consistent(internal,internal,int4)
 RETURNS bool
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 -- register the opclass for indexing (not as default)
 CREATE OPERATOR CLASS gist_hstore_ops
@@ -241,17 +241,17 @@ AS
 CREATE FUNCTION gin_extract_hstore(internal, internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE FUNCTION gin_extract_hstore_query(internal, internal, int2)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE FUNCTION gin_consistent_hstore(internal, int2, internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE OPERATOR CLASS gin_hstore_ops
 DEFAULT FOR TYPE hstore USING gin