(Util model) Attribute inherits from Attributable

Goal of this commit is to allow Attributes objects to store other
attributes. It is useful for example in the case of annotations are
added to an Actor:

    @annot(arg1="val1",arg2="val2") @annot2 myobject

become something like

    myobject.attributes = [
        Attribute(
            name="annot",
            attributes=[
                Attribute(name="arg1", stringValue="val1"),
                Attribute(name="arg2", stringValue="val2")
            ]
        ),
        Attribute(name="annot2")
    ]

in the resulting IR
(ir) Change the way Annotation are stored and serialized