Make sure that all model elements are annotatable

Annotations were initially designed as a way that we could extend metadata without introducing breaking changes. However, there are cases where model elements are not annotatable, which limits our ability to extend those elements.

For example, OData-1187 proposes extending navigation property bindings to specify that a particular path is recursive. We'd like to do this as an annotation, to avoid introducing an incompatability with 4.0, but NavigationPropertyBinding elements are (currently) not annotatable.

We should go through our model elements to identify those that are not currently annotatable, and see if we can come up with a way to make everything annotatable.

Hubert had some thoughts about making it easier to annotate model elements in our JSON format.