a major refactoring in the C and the Haskell runtimes. Note incompatible change in the PGF format!!!

The following are the outcomes:

   - Predef.nonExist is fully supported by both the Haskell and the C runtimes

   - Predef.BIND is now an internal compiler defined token. For now
     it behaves just as usual for the Haskell runtime, i.e. it generates &+.
     However, the special treatment will let us to handle it properly in 
     the C runtime.

   - This required a major change in the PGF format since both 
     nonExist and BIND may appear inside 'pre' and this was not supported
     before.

src/compiler/GF/Compile/Compute/AppPredefined.hs

@@ -84,6 +84,8 @@ primitives = Map.fromList
                                          [(Explicit,varL,typeType),(Explicit,identW,mkFunType [typeStr] typeStr),(Explicit,identW,Vr varL)] (Vr varL) []))) Nothing)
   , (cNonExist , ResOper (Just (noLoc (mkProd -- Str
                                          [] typeStr []))) Nothing)
+  , (cBIND     , ResOper (Just (noLoc (mkProd -- Str
+                                         [] typeStr []))) Nothing)
   ]
   where
     fun from to = oper (mkFunType from to)