Use Arc of adapter to execute queries.

[obi1kenobi]

Using trustfall in a web server was previously quite tricky because it required an Rc<Adapter> whereas servers tend to wrap everything inside Arc. The mismatch was even more striking since some of Trustfall's other input arguments were themselves wrapped in Arc.

In principle, Trustfall doesn't need to know whether it's passed an Rc or Arc of the adapter. There's a way to do this with generics, but it's a bit complex and the overhead is not worth it right now. Instead, we make evaluating queries always use an Arc to get a large usability and ergonomics win at a tiny perf cost. We can relatively easily win back more than enough performance by optimizing other things. Stay tuned!