Use transformers instead of forwarding reactables

[Dan Schultz]

Most of the transformation methods right now, i.e. flatMapLatest(), debounce(), are sub-classes of _ForwardingReactable.

A better approach is to create these as implementations of StreamTransformer. This utilizes some of the built in behavior of Dart and also opens the door for giving developers a choice of how they want to apply the transformation.

For example, if you only had a Dart stream, you could still utilize debounce by doing:

var stream = new Stream.fromIterable([1]);
stream.transform(new Debounce(new Duration(seconds: 1));