StreamClass.map<S, U>(source: Source<S>,fn: (value: Exclude<S, AnyFailure>,index: number) => U | PromiseLike<U>): StreamClass<Exclude<U, AnyFailure>,Extract<S, AnyFailure> | Extract<U, AnyFailure>>
StreamClass#map as a static: Stream.map(source, …) is
Stream.from(source).map(…).
await Stream.map([1, 2], (n) => n * 2).collect(); // [2, 4]