Add hint to AnonymousArgument.expression doc

See previous commit
Add Specifier (RedHat’s SpecifierExpression)
Add tests for Specifier backend conversion
Add ValueSpecification, Specification, Statement

No, I’m not starting on statements yet, I just need them as the
supertype of Specification.
Add backend tests for ValueSpecification + supertypes
Add SpecifiedArgument
Add tests for SpecifiedArgument backend conversion