Translating linearization functions to Haskell: move Haskell AST and pretty printer to GF.Haskell

For further separation of pretty printing concerns from conversion concerns,
the Haskell AST and pretty printer has been moved to its own module,
GF.Haskell, also allowing it to be reused in other places where Haskell
code is generated.

gf.cabal

@@ -184,6 +184,7 @@ Library
     GF.Compile.Multi
     GF.Compile.Optimize
     GF.Compile.PGFtoHaskell
+    GF.Haskell
     GF.Compile.ConcreteToHaskell
     GF.Compile.PGFtoJS
     GF.Compile.PGFtoLProlog