feature: create map, compactMap, filter, flatMap ObservableType with with object

[DevVenusK]

To facilitate chaining methods of Observable, a method with a with parameter has been overloaded.

When using withUnretained, it requires two different approaches.

1. Keep returning owner.

Observable.just<1>
  .withUnretained(self)
  .map { (owner, value) in
        return (owner, value * 2)
  }
  .map { (owner, value) in
        return (owner, value + 1)
  }

2. Continue using the withUnretained method.

Observable.just<1>
  .withUnretained(self)
  .map { (owner, value) in
         return (value * 2)
  }
  .withUnretained(self)
  .map { (owner, value) in
        return (value + 1)
  }

However, to eliminate such inconvenience, the method has been overloaded with a with parameter.

Observable.just<1>
  .map(with: self) { $1 * 2 }
  .map(with: self) { $1 + 1 }