Add interface classes (closes #57)

- AnyInterface (RedHat’s AnyInterface)
  - AnyInterfaceDefinition (RedHat’s InterfaceDefinition)
    - InterfaceDefinition
    - DynamicInterfaceDefinition

And as it turns out, the design with AnyInterfaceDefinition is very
beneficial for the RedHat AST conversion, as it helps avoid code
duplication.
Add tests for interface nodes