Use UnitId in the backend instead of Unit
In Cmm we can only have real units identified with an UnitId. Other units (on-the-fly instantiated units and holes) are only used in type-checking backpack sessions that don't produce Cmm.
In Cmm we can only have real units identified with an UnitId. Other units (on-the-fly instantiated units and holes) are only used in type-checking backpack sessions that don't produce Cmm.