Virtual predicates

Use #668 to store queries in database. Next, allow to register virtual predicates that refer these queries. Shapes optimizer will run a rewrite pass to detect predicates that are registered as virtual and will add a specified subquery to the tree. Will require #667 to be implemented.

Start by reviewing issues #668 and #667 to understand the query storage and prerequisite work. Then inspect the Shapes optimizer's rewrite pass and determine how registered virtual predicates should expand into subqueries; done means the registration and rewrite behavior are implemented and covered by relevant tests, though no test files are named here.

Written by the indexing model from the issue text.