Make UtEnumConstantModel and UtClassRefModel subclasses of UtReferenceModel

[dtim]

Description

Although Enum is a reference type, UtEnumConstantModel is not currently a subtype of UtReferenceModel and does not have any id that would allow the reference comparison of Enum values. As a result, we can't use Enum values as elements of generic collections (Map<K, V>, List<E> etc).

The intended solution:

• UtEnumConstantModel and UtClassRefModel should be subtypes of UtReferenceModel.
• The address assigned to a UtEnumConstantModel should be equal to one and only one address assigned to models of the static fields of the corresponding Enum class, and reference equality must hold.
• The addresses should be assigned to UtClassRefModel with respect to class equality condition: all models of the same class should have the same address, models of different classes should have different addresses,

Related issue: #230

Expected behavior

UtEnumConstantModel is a subtype of UtReferenceModel, and enum values can be used as elements of any generic collection.

Environment

The intended behavior does not depend on any specific environment.