Support functions for index opclasses should be immutable.

Found by running opr_sanity on contrib modules.

contrib/cube/cube.sql.in

@@ -263,37 +263,37 @@ CREATE OPERATOR ~ (
 CREATE OR REPLACE FUNCTION g_cube_consistent(internal,cube,int4)
 RETURNS bool
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE OR REPLACE FUNCTION g_cube_compress(internal)
 RETURNS internal 
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE OR REPLACE FUNCTION g_cube_decompress(internal)
 RETURNS internal 
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE OR REPLACE FUNCTION g_cube_penalty(internal,internal,internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C IMMUTABLE STRICT;
 
 CREATE OR REPLACE FUNCTION g_cube_picksplit(internal, internal)
 RETURNS internal
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE OR REPLACE FUNCTION g_cube_union(internal, internal)
 RETURNS cube 
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 CREATE OR REPLACE FUNCTION g_cube_same(cube, cube, internal)
 RETURNS internal 
 AS 'MODULE_PATHNAME'
-LANGUAGE C;
+LANGUAGE C IMMUTABLE;
 
 
 -- Create the operator classes for indexing