Use a more space/time-efficient representation for the eval cache

src/libexpr/eval.cc

@@ -351,6 +351,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sOutputHashMode(symbols.create("outputHashMode"))
     , sDescription(symbols.create("description"))
     , sSelf(symbols.create("self"))
+    , sEpsilon(symbols.create(""))
     , repair(NoRepair)
     , store(store)
     , baseEnv(allocEnv(128))

src/libexpr/eval.hh

@@ -75,7 +75,7 @@ public:
         sFile, sLine, sColumn, sFunctor, sToString,
         sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
         sOutputHash, sOutputHashAlgo, sOutputHashMode,
-        sDescription, sSelf;
+        sDescription, sSelf, sEpsilon;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they