Add backward compatibility for reading old PGF files

Some backwards incompatible changes were made to the PGF file format after
the release of GF 3.5. This patch adds a module for reading PGF files in the
old format.

This means that old PGF files on the grammaticalframework.org server will
continue to work after we install the latest version of GF.

src/runtime/haskell/PGF/Binary.hs

@@ -1,8 +1,9 @@
-module PGF.Binary where
+module PGF.Binary(putSplitAbs) where
 
 import PGF.CId
 import PGF.Data
 import PGF.Optimize
+import qualified PGF.OldBinary as Old
 import Data.Binary
 import Data.Binary.Put
 import Data.Binary.Get
@@ -14,7 +15,7 @@ import qualified Data.IntMap as IntMap
 import Control.Monad
 
 pgfMajorVersion, pgfMinorVersion :: Word16
-(pgfMajorVersion, pgfMinorVersion) = (2,0)
+version@(pgfMajorVersion, pgfMinorVersion) = (2,0)
 
 instance Binary PGF where
   put pgf = do putWord16be pgfMajorVersion
@@ -24,7 +25,11 @@ instance Binary PGF where
                put (concretes pgf)
   get = do v1 <- getWord16be
            v2 <- getWord16be
-           gflags <- get
+           case (v1,v2) of
+             v | v==version -> getPGF'
+               | v==Old.version -> Old.getPGF'
+
+getPGF'=do gflags <- get
            (absname,abstract) <- get
            concretes <- get
            return $ updateProductionIndices $