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:

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.