Improved RowToJS performance by removing Napi::String::New instan…

…tiation

refs https://github.com/nodejs/node-addon-api/blob/main/doc/object.md#set

- according to the node-addon-api docs, you can set various things as
  the key for a Napi::Object, including a std::string ref
- instantiating a `Napi::String::New` is quite heavy, especially when
  we're doing it for every row we return, so we can avoid doing that and
  speed up the function
- locally, this speeds up the benchmark by 5-15% (a lot of variance) but YMMV

src/statement.cc

@@ -814,7 +814,7 @@ Napi::Value Statement::RowToJS(Napi::Env env, Row* row) {
             } break;
         }
 
-        (result).Set(Napi::String::New(env, field->name.c_str()), value);
+        result.Set(field->name, value);
     }
 
     return scope.Escape(result);