haskell-bind/PGF2.hsc: unexport functions that break referential transparency

loadConcr, unloadConcr and addLiteral modify the Concr structure as a side
effect. This means that other functions with a Concr argument (e.g. parse
and linearize) are no longer pure.

Possible solutions:

1. Don't try to hide the imperative nature of the C run-time system: remove
   all uses of unsafePerformIO and let all functions operate in the IO monad.

2. Don't export functions with side effects. Perhaps the desired functionality
   of loadConcr, unloadConcr and addLiteral can be folded into readPGF.
   The Concr structures can then treaded as immutable after after the
   readPGF function returns...

src/runtime/haskell-bind/PGF2.hsc

@@ -14,9 +14,8 @@
 
 module PGF2 (-- * PGF
              PGF,readPGF,abstractName,startCat,
-             loadConcr,unloadConcr,
              -- * Concrete syntax
-             Concr,languages,parse,linearize,addLiteral,
+             Concr,languages,parse,linearize,
              -- * Trees
              Expr,readExpr,showExpr,unApp,
              -- * Morphology