Exercise JSON compilation in MyriaL tests

* Exercise the JSON compilation logic as part of Myrial unit tests.  We don't do anything with the output other than verify its non-emptyness.  Certain tests are exempted due to a lack of a backend implementation.

* Fix/add compilation for various operators

* Map CountAll to Count($0), which is dodgy if we ever support null values.

* Change the semantics of the difference and intersection operator to have set semantics; this is in keeping with SQL and the Myrial Difference operator.