* feat: add function 'pg_catalog.pg_table_is_visible'q
* feat: add 'pg_class' and 'pg_namespace', now we can run '\d' and '\dt'!
* refactor: move memory_table::tables to utils::tables
* refactor: move out predicate to system_schema to reuse it
* feat: predicates pushdown
* test: add pg_namespace, pg_class related sqlness test
* fix: typos and license header
* fix: sqlness test
* refactor: use `expect` instead of `unwrap` here
* refactor: remove the `information_schema::utils` mod
* doc: make the comment in pg_get_userbyid more precise
* doc: add TODO and comment in pg_catalog
* fix: typo
* fix: sqlness
* doc: change to comment on PGClassBuilder to TODO