New planner rule added to push predicates down in the query representation

[alecgrieser]

This introduces a new query planner rule that will be a part of a larger framework to canonicalize query representations. The rule pushes down predicates to ensure that they appear as low in the tree as possible. So, for example, suppose you had a query like:

SELECT a, b, c
   FROM (SELECT a, b, c, d FROM T)
   WHERE d = ?
   ORDER BY a, b

This will produce a new query expression where the d = ? is pushed lower in the graph, something like:

SELECT a, b, c
   FROM (SELECT a, b, c, d FROM T where d = ?)
   ORDER BY a, b

This simplification helps us during planning to push down predicate execution to the lowest part of the graph where they can go.

Note that for the moment, this rule has not been added to the default PlanningRuleSet. The reason for this is that some of the rewrite rules will have weird overlap with some of the other rules, and so we want to rewrite the query in one phase and then plan and optimize it in another. The framework for that is not there yet

[alecgrieser]

This change here shows how the new utility methods can make the existing tests more concise. I only updated this one and testSimpleExistentialPredicateOnSimpleIndex as a proof of concept, as shortening the rest isn't really a goal in this PR, but you can get similar results with the other tests here

[normen662]

Want to create an issue for that?