Map method flow tracing

The Map object is created by using the mediator mode. The Map object also inherits from Producer class and has the function of adding subscribers.

When the Map is subscribed, create MapSink, store AnomymousObserver, and call subscribe (Observer) method of sourceObservable with MapSink as parameter.

At the same time, MapSink has the implementation of on (:) method, which performs _transform operation on the value of the sequence by invoking layer upon layer callback, and calls AnomymousObserver’s on () method with the result as parameter.