Represents inferred structural type. A type may be either primitive type
 (one of those listed by `primitiveTypes`) or it can be collection,
 (named) record and heterogeneous type. We also have `Null` type (which is
 a subtype of all non-primitive types) and universal `Top` type.

  * For collection, we infer the types of different things that appear in
    the collection and how many times they do.

  * A heterogeneous type (sum type) is simply a choice containing one
    of multiple different possibilities

 Why is collection not simply a list of Heterogeneous types? If we used that
 we would lose information about multiplicity and so we would not be able
 to generate nicer types!