Fixes for GHC 8.4.1 compatibility

* In GHC 8.4.1, the operator <> has become a method of the Semigroup class
  and is exported from the Prelude. This is unfortunate, since <> is also
  exported from the standard library module Text.PrettyPrint, so in any
  module that defines a pretty printer, there is likely to be an ambiguity.

  This affects ~18 modules in GF. Solution:

    import Prelude hiding (<>)

  This works also in older versions of GHC, since GHC does't complain if
  you hide something that doesn't exists.

* In GHC 8.4.1, Semigroup has become a superclass of Monoid. This means
  that anywhere you define an instance of the Monoid class you also have to
  define an instance in the Semigroup class.

  This affects Data.Binary.Builder in GF. Solution: conditionally define
  a Semigroup instance if compiling with base>=4.11 (ghc>=8.4.1)

src/runtime/haskell-bind/PGF2.hsc

@@ -82,7 +82,7 @@ module PGF2 (-- * PGF
              LiteralCallback,literalCallbacks
             ) where
 
-import Prelude hiding (fromEnum)
+import Prelude hiding (fromEnum,(<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 import Control.Exception(Exception,throwIO)
 import Control.Monad(forM_)
 import System.IO.Unsafe(unsafePerformIO,unsafeInterleaveIO)

src/runtime/haskell/Data/Binary/Builder.hs

@@ -100,6 +100,11 @@ newtype Builder = Builder {
         runBuilder :: (Buffer -> [S.ByteString]) -> Buffer -> [S.ByteString]
     }
 
+#if MIN_VERSION_base(4,11,0)
+instance Semigroup Builder where
+  (<>) = append
+#endif
+
 instance Monoid Builder where
     mempty  = empty
     {-# INLINE mempty #-}

src/runtime/haskell/PGF/ByteCode.hs

@@ -2,7 +2,7 @@ module PGF.ByteCode(Literal(..),
                     CodeLabel, Instr(..), IVal(..), TailInfo(..),
                     ppLit, ppCode, ppInstr
                    ) where
-
+import Prelude hiding ((<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 import PGF.CId
 import Text.PrettyPrint
 

src/runtime/haskell/PGF/Macros.hs

@@ -1,4 +1,5 @@
 module PGF.Macros where
+import Prelude hiding ((<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 
 import PGF.CId
 import PGF.Data

src/runtime/haskell/PGF/Printer.hs

@@ -1,5 +1,6 @@
 {-# LANGUAGE FlexibleContexts #-}
 module PGF.Printer (ppPGF,ppCat,ppFId,ppFunId,ppSeqId,ppSeq,ppFun) where
+import Prelude hiding ((<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 
 import PGF.CId
 import PGF.Data

src/runtime/haskell/PGF/VisualizeTree.hs

@@ -23,6 +23,7 @@ module PGF.VisualizeTree
              , gizaAlignment
              , conlls2latexDoc
              ) where
+import Prelude hiding ((<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 
 import PGF.CId (wildCId,showCId,ppCId,mkCId) --CId,pCId,
 import PGF.Data