StreamClass.flatMap<S, U>(): StreamClass<Exclude<U, AnyFailure>,Extract<S, AnyFailure> | Extract<U, AnyFailure>>
StreamClass#flatMap as a static: Stream.flatMap(source, …) is
Stream.from(source).flatMap(…).
await Stream.flatMap([1, 2], (n) => [n, n]).collect(); // [1, 1, 2, 2]