refactor - introducing internal class Observation

[beeth0ven]

Introducing internal class Observation. Conceptly Observation can be considered as iterator to Observable.

Relationship between Iterable and Iterator

class Iterator<T> { ... }

class Iterable<T> {
  
  Iterator<T> get iterator {
    return Iterator(...);
  }
}

Relationship between Observable and Observation

class Observation<T> implements Disposable { ... }

class Observable<T> {

  Disposable observe(OnData<T> onData) {
    return Observation(...);
  }
}

Plan to refactor part of Observable* implementation using Observation:

• refactor - add internal class Observation #160
• refactor - reimplements ObservableCombine using Observation #159
• refactor - reimplements ObservableCreate using Observation #163
• refactor - reimplements ObservableDistinct using Observation #165
• refactor - reimplements ObservableMulticast using Observation #166
• refactor - reimplements ObservableSkip using Observation #167