Move implementation of transformation methods to Reactable

[danschultz]

Look into moving the implementation of transformation methods to Reactable. This'll make it easier to sub-class Reactable.

class Stream<T> {
  ...
}

abstract class Reactable<T, R extends Reactable> extends Stream<T> {
  R map(block(T value)) {
    return createInstance(super.map(block));
  }

  R createInstance(Stream<T> stream);
}

class Property<T> extends Reactable<T, Property<T>> {
  Property<T> createInstance(Stream<T> stream) {
    return new Property.fromStream(stream);
  }
}