cache more often-used symbols for primops

there's a few symbols in primops we can create once and pick them out of
EvalState afterwards instead of creating them every time we need them. this
gives almost 1% speedup to an uncached nix search.

src/libexpr/eval.hh

@@ -80,7 +80,8 @@ public:
         sContentAddressed,
         sOutputHash, sOutputHashAlgo, sOutputHashMode,
         sRecurseForDerivations,
-        sDescription, sSelf, sEpsilon;
+        sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
+        sPrefix;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they