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/Predef.hs

@@ -78,7 +78,7 @@ predefList =
      (cError,Error),
      -- Canonical values:
      (cPBool,PBool),(cPFalse,PFalse),(cPTrue,PTrue),(cInt,Int),
-     (cInts,Ints),(cNonExist,NonExist)]
+     (cInts,Ints),(cNonExist,NonExist),(cBIND,BIND)]
     --- add more functions!!!
 
 delta f vs =
@@ -106,6 +106,7 @@ delta f vs =
       PFalse  -> canonical
       PTrue   -> canonical
       NonExist-> canonical
+      BIND    -> canonical
   where
     canonical = delay
     delay = return (VApp f vs) -- wrong number of arguments