Add core-def form for annotated definitions

[AlexKnauth]

This is the solution discussed in https://github.com/lexi-lambda/hackett/issues/73#issuecomment-390085946 for a Core-def form that's always annotated.

This will allow our Module/Functor system to catch these when it partially expands the definitions in a module, so that it can pull out the ids and types into the signature of the module.