Compute effectively functional interfaces

[khatchad]

We saw with #35 that functional interfaces should not be refactored, i.e., they should fail preconditions. However, for #35, we are simply using the @FunctionalInterface annotation to determine if a particular interface is functional. However, this annotation just serves as a hint to the compiler to verify the constraints that make up functional interfaces. Developers are free to create and use functional interfaces without this annotation. As such, refactoring these interfaces will cause problems, particularly when they are used in lambda expressions and method references.

This task is to expand the edu.cuny.citytech.defaultrefactoring.core.refactorings.MigrateSkeletalImplementationToInterfaceRefactoringProcessor.isInterfaceFunctional(IType) method to find functional interfaces not having the @FunctionalInterface annotation.