Compute realizability status
A set of all fields that have already been checked. Used to avoid infinite recursions when analyzing recursive types.
tp has good bounds, a
pointing to a bad bounds member otherwise.
Is symbol's definitition a lazy val? (note we exclude modules here, because their realizability is ensured separately)
Realizable if all of
tp's non-struct fields have realizable types,
HasProblemField instance pointing to a bad field otherwise.
The realizability status of given type