a partial support for def rules in the C runtime

The def rules are now compiled to byte code by the compiler and then to
native code by the JIT compiler in the runtime. Not all constructions
are implemented yet. The partial implementation is now in the repository
but it is not activated by default since this requires changes in the
PGF format. I will enable it only after it is complete.

src/runtime/haskell/PGF/Internal.hs

@@ -11,6 +11,7 @@ import PGF.Macros as Internal
 import PGF.Optimize as Internal
 import PGF.Printer as Internal
 import PGF.Utilities as Internal
+import PGF.ByteCode as Internal
 
 import Data.Binary as Internal
 import Data.Binary.Get as Internal