The root cause of #34 was that we took a type argument which was an existential type. These are returned as type bounds, which make no sense in the calling context. To avoid that problem in the future, `typeArgs`
got renamed to `argInfos`, so it is clear we get an info, not necessarily a value type. There are
also added method `argTypes`, `argTypesLo`, `argTypesHi`, which return a type, but either throw an exception or return a lower/upper approximation of the argument is an existential type.
There's another issue that the existential type only arose when compiling the same couple fo files the seciond time. We need to chase that one down separately.