Changes:

1) Add "isdistinctfrom" and "notdistinctfrom" native expressions.

2) Add "IS [NOT] DISTINCT FROM" to SQL. It uses the new native expressions
   when generating expressions, and is treated the same as equals and
   not-equals when generating native filters on literals.

3) Update join matchers to have an "includeNull" parameter that determines
   whether we are operating in "equals" mode or "is not distinct from"
   mode.

- Add ARRAY handling to "notdistinctfrom" and "isdistinctfrom".
- Include null in pushed-down filters when using "notdistinctfrom" in a join.

Other changes:
- Adjust join filter analyzer to more explicitly use InDimFilter's ValuesSets,
  relying less on remembering to get it right to avoid copies.