Fix select get owned

[ytanimura]

SCHEMA test_schema;
  ENTITY a;
    x: REAL;
  END_ENTITY;

  ENTITY b;
    y: INTEGER;
  END_ENTITY;

  ENTITY c;
    z: STRING;
  END_ENTITY;

  TYPE sup = SELECT (a, b);
  END_TYPE;

  TYPE sup_sup = SELECT (sup, c);
  END_TYPE;
END_SCHEMA;

With the current implementation, given the module defined in the express file above, sup_sup cannot get the proper value by EntityTable::get_owned. Instead of using rustep::tables::get_owned directly, EntityTable::get_owned must be used recursively.