Add AST annotations

[TravisCardwell]

Issue: #256

This WIP implementation has one pass named Parsed and unit annotations. We probably do not want to merge without implementing realistic passes/annotations, so this PR is created as a draft.

Note that many tests currently fail because the fixtures have not been updated to include the annotations.

[edsko]

After discussing this with @TravisCardwell , we decided to go a slightly different route, which would make the "infrastructure" (such as it is) more generally applicable to all phases in hs-bindgen:

-- | All passes in hs-bindgen (from parsing C to final generation)
data Pass = ...

-- closed!
type family Ann (p :: Pass) (a :: Star) where
  Ann Parsed a = AnnParsed a
  Ann p a = () -- All annotations of all types default to ()

type family AnnParsed a -- could be open or closed, doesn't matter
  
deriving stock instance (forall p. Show (Ann "Foo" p)) => Show (Foo p)