Flatten Lens: Infer Unique and Foreign Key Constraints

[DRovara]

Added functionality to infer integrity constraints for the Flatten Lens:

• UC: If a position argument pos is included in the Lens, and any of the base relation's UCs c is fully included, then c + pos is a unique constraint.
• FK: If any of the base relation's UCs is fully included, then we can use it as a foreign key to the base relation.

[bcogrel]

Thanks!