Custom Filter generator (top-level), which enforces a (sane) limit on the depth of the generated filters.

Generates an arbitrary JSValue. We do this via a function a not via arbitrary instance since that would require us to define an arbitrary for JSValue, which can be recursive, entering the usual problems with that; so we only generate the base types, not the recursive ones, and not JSNull, which we can't use in a RSNormal ResultEntry.