Fix string type lvalue return type

src/codegen.cpp

@@ -49,7 +49,9 @@ namespace AST {
         if (scope.is_lvalue) {
             return codegen::StackValue{
                 global_str,
-                std::unique_ptr<types::Type>{stack_type},
+                std::unique_ptr<types::Type>{
+                    new types::PointerType { std::shared_ptr<types::Type> {stack_type } }
+                },
             };
         }
         else {