one-to-one and many-to-many relations, relations by join columns

* An orm slot now supports one-to-one and many-to-many relations.

* For one-to-one and many-to-one/one-to-many relations in addition
  to relation tables, join columns can be used.

* One-to-many slots now support being ordered. This requires the
  use of a relation table, with a number field called INDEX.

* Added code that checks the integrity of the arguments to an
  ABSTRACT-ORM-CLASS slot.