Add missing function abs (interval)

On a newly set up system there are 7 types with a unary minus operator
defined, but only 6 of them have an abs function:

postgres=# \df abs
List of functions
Schema | Name | Result data type | Argument data types | Type
------------+------+------------------+---------------------+------
pg_catalog | abs | bigint | bigint | func
pg_catalog | abs | double precision | double precision | func
pg_catalog | abs | integer | integer | func
pg_catalog | abs | numeric | numeric | func
pg_catalog | abs | real | real | func
pg_catalog | abs | smallint | smallint | func
(6 rows)

I now have the following definition in my database:

CREATE OR REPLACE FUNCTION abs (
p interval
) RETURNS interval
LANGUAGE SQL IMMUTABLE STRICT
SET search_path FROM CURRENT
AS $$
SELECT GREATEST (p, -p)
$$;
COMMENT ON FUNCTION abs (interval) IS 'absolute value';

Would a patch to add a function with this behaviour to the initial database
be welcome?

If so, should I implement it essentially like the above, or as an internal
function? I've noticed that even when it seems like it might be reasonable
to implement a built-in function as an SQL function they tend to be
internal.