Eliminated OnSubscribeFunc in favor of IObservable itself.

The purpose of OnSubscribeFunc was to provide a simple, composable API for observability. IObservable already does that, and with the same method signature, so there’s no need for a second API for the same thing. There are two benefits for the API:
1. The syntax for composing operations is cleaner, with less overhead in syntax and in object count. Instead of Observable.create(op(create(arg1), create(arg2), create(arg3))), you can now write simple op(arg1, arg2, arg3).
2. The Operation classes are more consistent with one another. Previously, some of them provided actual Observables, and others merely OnSubscribeFunc. Which one was expected when wasn’t particularly clear, and it was up to the caller to wrap or unwrap the object as needed. Now, every Operation provides IObservables of one concrete type or another.