template-haskell: Add putDoc, getDoc and friends

This MR is an implementation of @mpickering's comment in #5467 (closed) It adds two new functions, addDoc and getDoc that can attach documentation to module headers, declarations, function arguments and class/family instances, as well as retrieve it, provided that Opt_Haddock is enabled. This part extends template-haskell and GHC's opt-haddock support, since Haddock doesn't seem to use the .hi files just yet there is a separate patch that integrates this into Haddock at this branch

Things to do:

• Tests
• Is String the right type for docs?
• Add combinators
• Take a closer look at getDoc (should probably allow it to lookup docs that were added via addDoc
• Does that IORef in TcGblEnv need to be released?
• What happens when Opt_Haddock isn't on?
• How does this work with things defined outside of the module?
• Rename addDoc to setDoc