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.cc

@@ -425,6 +425,11 @@ EvalState::EvalState(
     , sDescription(symbols.create("description"))
     , sSelf(symbols.create("self"))
     , sEpsilon(symbols.create(""))
+    , sStartSet(symbols.create("startSet"))
+    , sOperator(symbols.create("operator"))
+    , sKey(symbols.create("key"))
+    , sPath(symbols.create("path"))
+    , sPrefix(symbols.create("prefix"))
     , repair(NoRepair)
     , emptyBindings(0)
     , store(store)