StreamCloser<T> class Null safety

A StreamTransformer that allows the caller to forcibly close the transformed Stream.

When close is called, any stream (or streams) transformed by this transformer that haven't already completed or been cancelled will emit a done event and cancel their underlying subscriptions.

Note that unlike most StreamTransformers, each instance of StreamCloser has its own state (whether or not it's been closed), so it's a good idea to construct a new one for each use unless you need to close multiple streams at the same time.