* Optimise null-ary term builders. Also declare all term builder

functions as pure, which might improve performance a bit.

src/libexpr/eval.hh

@@ -29,7 +29,6 @@ struct EvalState
     DrvRoots drvRoots;
     DrvHashes drvHashes; /* normalised derivation hashes */
     SrcToStore srcToStore; 
-    Expr blackHole;
 
     unsigned int nrEvaluated;
     unsigned int nrCached;