Refactor type hierarchy logic

Most of the type hierarchy logic is in `eval/TypeChecker.java`, which powers the dynamic type checker. We should generalize and move this logic into `StarlarkType` or related helpers, which can be accessed by the static type checker.

We need to take care to implement correct gradual typing behavior w.r.t. `Any` in the type hierarchy, i.e. a "consistent subtype of" relation.