Represents the kind of the anonymous structure type.
A closed structure is considered complete. That is, no further fields can be added to it.
An open structure allows having additional fields added to it, which is used during type inference. It is closed upon unification.
A const structure is one that appears directly in syntax. It cannot be assigned to a smaller structure type (that is, it does not allow structural sub-typing).
Represents a structure which extends one or multiple structures defined