Add environment variable NIX_SHOW_SYMBOLS for dumping the symbol table

src/libexpr/eval.cc

@@ -1811,6 +1811,7 @@ void EvalState::printStats()
             gc.attr("totalBytes", totalBytes);
         }
 #endif
+
         if (countCalls) {
             {
                 auto obj = topObj.object("primops");
@@ -1846,6 +1847,11 @@ void EvalState::printStats()
                 }
             }
         }
+
+        if (getEnv("NIX_SHOW_SYMBOLS", "0") != "0") {
+            auto list = topObj.list("symbols");
+            symbols.dump([&](const std::string & s) { list.elem(s); });
+        }
     }
 }