Do pretty-printing of TyThings via IfaceDecl (Trac #7730)

All the initial work on this was done fy 'archblob' (fcsernik@gmail.com);
thank you!

I reviewed the patch, started some tidying, up and then ended up in a huge
swamp of changes, not all of which I can remember now.  But:

* To suppress kind arguments when we have -fno-print-explicit-kinds,
    - IfaceTyConApp argument types are in a tagged list IfaceTcArgs

* To allow overloaded types to be printed with =>, add IfaceDFunTy to IfaceType.

* When printing data/type family instances for the user, I've made them
  print out an informative RHS, which is a new feature. Thus
        ghci> info T
        data family T a
        data instance T Int = T1 Int Int
        data instance T Bool = T2

* In implementation terms, pprIfaceDecl has just one "context" argument,
  of type IfaceSyn.ShowSub, which says
       - How to print the binders of the decl
         see note [Printing IfaceDecl binders] in IfaceSyn
       - Which sub-comoponents (eg constructors) to print

* Moved FastStringEnv from RnEnv to OccName

It all took a ridiculously long time to do.  But it's done!

Author: Simon Peyton Jones

compiler/basicTypes/OccName.lhs

@@ -102,7 +102,10 @@ module OccName (
 	-- * Lexical characteristics of Haskell names
 	isLexCon, isLexVar, isLexId, isLexSym,
 	isLexConId, isLexConSym, isLexVarId, isLexVarSym,
-	startsVarSym, startsVarId, startsConSym, startsConId
+	startsVarSym, startsVarId, startsConSym, startsConId,
+
+        -- FsEnv
+        FastStringEnv, emptyFsEnv, lookupFsEnv, extendFsEnv, mkFsEnv
     ) where
 
 import Util
@@ -117,6 +120,29 @@ import Data.Char
 import Data.Data
 \end{code}
 
+%************************************************************************
+%*									*
+              FastStringEnv
+%*									*
+%************************************************************************
+
+FastStringEnv can't be in FastString becuase the env depends on UniqFM
+
+\begin{code}
+type FastStringEnv a = UniqFM a         -- Keyed by FastString
+
+
+emptyFsEnv  :: FastStringEnv a
+lookupFsEnv :: FastStringEnv a -> FastString -> Maybe a
+extendFsEnv :: FastStringEnv a -> FastString -> a -> FastStringEnv a
+mkFsEnv     :: [(FastString,a)] -> FastStringEnv a
+
+emptyFsEnv  = emptyUFM
+lookupFsEnv = lookupUFM
+extendFsEnv = addToUFM
+mkFsEnv     = listToUFM
+\end{code}
+
 %************************************************************************
 %*									*
 \subsection{Name space}
@@ -246,6 +272,9 @@ instance Data OccName where
   toConstr _   = abstractConstr "OccName"
   gunfold _ _  = error "gunfold"
   dataTypeOf _ = mkNoRepType "OccName"
+
+instance HasOccName OccName where
+  occName = id
 \end{code}