Is your feature request related to a problem? Please describe.
Our engine integration tests now run on a limited sample of hand-crafted JSONs and queries, as well as the Wikidata JSONs (again with hand-picked queries). A proptest suite generating input JSONs and queries would make the library much more fail-proof.

Describe the solution you'd like
This is a complex design space. An iterative approach might work here -- create very limited inputs (only certain types of JSON trees and queries) to start off and then continue expanding that in follow-up issues/PRs.

Additional context
To get started, see the proptest book.