Coercion logic does not handle Enum type correctly.

[hatyo]

In Cascades, the logic for handling coercion of Type.Enum is missing, and it should be fixed by adding basic no-op rules that permit the following no-op coercions:
For a given Enum E1:

• nullable E1 to nullable E1 (enum to itself)
• not nullable E1 to not nullable E1 (enum to itself)
• not nullable E1 to nullable E1 (this is fine since the nullable E1's domain can accommodate for all elements in E1.