Functions, aggregates, and user types

Querying system_schema tables directly to get information about user-defined functions, aggregates, and user types.

Currently, the system_schema tables are the only method of displaying information about user-defined functions, aggregates, and user types.

Procedure

Show all user-defined functions in the system_schema.functions table.
```
SELECT *
FROM system_schema.functions;
```
Show all user-defined aggregates in the system_schema.aggregates table.
```
SELECT *
FROM system_schema.aggregates;
```
Show all user-defined types in the system_schema.types table.
```
SELECT *
FROM system_schema.types;
```