Implement the UNNEST relational operator, and various improvements to ARRAY and MULTISET data types.

Implement CollectRel and UncollectRel relational operators, SLICE and ELEMENT functions, make UNNEST apply to both ARRAY and MULTISET.

Document UNNEST, ELEMENT, CARDINALITY and TIMESTAMP WITH TIME ZONE in reference guide.
Disable MongoDB a test, pending https://github.com/julianhyde/optiq/issues/270.