Improve level polymorphism

We could upgrade `Set` to be of type `Type a → Type b`. The downside is that we'd have to reformulate `unions` to look more like the monadic `bind` operation.

The main benefit of this would be that the `Predicates` model would then not require `--type-in-type` anymore. I don't know if there are any downsides.

Start by locating the definitions of Set, unions, and the Predicates model, then inspect how --type-in-type is currently required. Evaluate the proposed Type a → Type b formulation and the corresponding unions redesign. Done means the polymorphic level change is implemented, unions remain usable, and Predicates no longer requires the flag without introducing regressions.

Written by the indexing model from the issue text.