Make RPITIT work across crates

[compiler-errors]

I'm pretty sure that these don't work cross-crate yet. If they do, then a test addition would at least be helpful.

1. Serialize and deserialize the results from the compare_predicates_and_trait_impl_trait_tys query like other separate_provide_extern queries.
2. Add a UI test with an aux-build crate.

[compiler-errors]

hold up, claiming this for now as I rework some stuff.