Revert the enum struct change

Not a regular git revert as there have been many merges and things.

src/libexpr/parser.y

@@ -643,7 +643,7 @@ Expr * EvalState::parseExprFromString(std::string_view s, const Path & basePath)
 
 Expr * EvalState::parseStdin()
 {
-    //Activity act(*logger, Verbosity::Talkative, format("parsing standard input"));
+    //Activity act(*logger, lvlTalkative, format("parsing standard input"));
     return parseExprFromString(drainFD(0), absPath("."));
 }