Update existential subquery CIP

[hvub]

This PR updates CIP2015-05-13 EXISTS (rendered view and in conjunction the grammar as well as TCK scenarios.

More specifically, the PR

• Adds a production for ExistentialSubquery
• Adds the ExistentialSubquery as alternative to Atom
• Renames the CaseAlternatives to CaseAlternative, since is represent only a single alternative
• Reuse the Where in the production PatternComprehension
• Removes feature on graph element property existence tests (Graph9 — as a consequence, Graph10 becomes Graph9)) and map field existence tests (Map4)
• Replaces exists(<X>) with <X> IS NOT NULL on remaining occurrences in TCK scenarios
• Adds/extends scenarios on <X> IS [NOT] NULL in Null1 and Null2
• Adds/extends scenarios on property/field access in Graph6 and Map1
• Add scenarios to on property/field existence test with ... IN keys(...) to Graph8 and Map3

[Mats-SX]

@hvub Great improvements! Thanks a lot for addressing my comments.