(PUP-992) Add support for Array as operand in relationships

This adds support for using Arrays with references as operands
in a relationship.
(PUP-994) Ensure relationship operands have uniq content

The relationship operands when containing duplicate entries
caused multiple relationships to be formed between the same entries.
Ensuring that input to the compiler is free of duplicates reduces the
amount of work at a later stage.
(maint) Fix typo in example title
Merge pull request #2145 from hlindberg/issues/relationship-operator-improvements

(PUP-992, 994) Fix issues with future evaluator relationship operator