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.
PGF exports the public, stable API.
PGF.Internal exports additional things needed in the GF compiler & shell,
including the nonstardard version of Data.Binary.
