Add a built in function to determine if a user is in a given role

It is possible to get the user() and effective_user(), however it is not possible to determine the users role.

Implement a mechanism to test that a user is within a certain role.
This function should return inline and be efficient.

Should be testable within a WHERE clause, providing per row-filtering within a view or query.

Alternatively provide a mechanism where the SHOW CURRENT ROLES function returns a table that can be used inline as a join with other tables.