#593 Add CIF annotation providers and use them to perform additional type checking

• This one is a bit more interesting than the previous ones. I think I came up with a rather nice and versatile design.
• Annotation providers are registered using a new extension point. They must be identified by the annotation name, and the implementation details (plugin/class).
• The class must implement a base class. It has documentation on the assumptions and guarantees.
• The type checker searches for a registered provider for the annotation, and then checks it using that provider.
• Best to review per commit.

Addresses #593