move identity function to utils

src/utils.ts

@@ -12,6 +12,10 @@ export function asArray<T>(iterable: Iterable<T>) {
   return Array.isArray(iterable) ? <T[]>iterable : Array.from(iterable);
 }
 
+export function identity<T>(obj: T) {
+  return obj;
+}
+
 class WrappedAsyncIterator<T> implements AsyncIterable<T> {
   readonly #iterator: AsyncIterator<T>;