Allow stored functions in CHECK constraints

We should be able to use stored functions in CHECK constraints:

CREATE OR REPLACE TABLE t (

    email VARCHAR(100) NOT NULL CHECK (is_email(email))

);

PostgreSQL and SQL Server allow this. I didn't check other DBMSs.

PostgreSQL assumes that the function logic never changes. If you replace the function with a new version, it won't notice if CHECK constraints fail. I think that this is a reasonable limitation.

Since MariaDB has a [NOT] DETERMINISTIC clause, it would also be reasonable to only allow DETERMINISTIC functions in CHECK constraints.