#602 Expressions - immutable parameter binding, pruning unbound conditions

[m-dzianishchyts]

• Remove ExpRoot node, so there is also no 'template' property (toString() can be used).
• Make parameter binding work with all expression nodes.
• Prune unused conditions during parameter binding.

[andrus]

Thanks for the PR. This feature is a cornerstone of all our upcoming expression evolution. I was already able to start on the first cut of the public expression factory API (#643). A few related notes:

• There were a few bugs related to handling ExpScalarLists that I fixed
• I started refactoring of AbstractExpTest subclasses. Parameterizing locally within a specific test (using @ValueSource and @CsvSource instead of @MethodSource) produces somewhat shorter and more transparent tests.
• We need to simplify the output Exp.toString() (#644)
• We need to tweak the exp structures for full immutability (#645). I don't have a full solution yet, but we'll work towards its step-by-step.