Unify "joins" and "get predicates" using existential subqueries

[gcampax]

The current distinction of join vs get predicate is somewhat arbitrary as it depends on the direction that the connection between the table goes. The syntax is inconsistent, and get predicates are limited to only filters.
Let's clean that up, and let's use subquery syntax as that more closely matches the semantics of being a kind of filter that also looks at a different table. Also subqueries are clearly not commutative, unlike joins.

Strawman syntax:

@org.schema.Restaurant.Restaurant() filter exists (@org.schema.Restaurant.Review() filter author == "bob" && in_array(id, review))

and

@org.schema.Restaurant.Restaurant() filter id in ([restaurant] of (@org.schema.Review() filter author=="bob"))

(depending on the direction of the relation, restaurant -> review or review -> restaurant)

[gcampax]

Done in #310 , with some leftovers in #316 and #317