PGF library: expose only PGF and PGF.Internal instead of all modules

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

src/runtime/haskell-bind/PGF2.hsc

@@ -1,4 +1,13 @@
 {-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-}
+-------------------------------------------------
+-- |
+-- Maintainer  : Krasimir Angelov
+-- Stability   : stable
+-- Portability : portable
+--
+-- This is the Haskell binding to the C run-time system for
+-- loading and interpreting grammars compiled in Portable Grammar Format (PGF).
+-------------------------------------------------
 #include <pgf/pgf.h>
 #include <gu/enum.h>
 #include <gu/exn.h>