4.x: Make Never<TResult> singleton.

[akarnokd]

This PR turns the operator of Never() into a singleton instance as, in theory, nothing should rely on the identity of it (i.e., something expecting Never() != Never()).

Unfortunately, there is something in Rx.NET itself that relies on object identity: the Join pattern's use of Dictionary where the key is the source observable:

Plan.cs#L96

which would crash And:

System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
System.Reactive.Joins.ActivePlan.AddJoinObserver(IJoinObserver joinObserver)
System.Reactive.Joins.ActivePlan`2..ctor(JoinObserver`1 first, JoinObserver`1 second, Action`2 onNext, Action onCompleted)
System.Reactive.Joins.Plan`3.Activate(Dictionary`2 externalSubscriptions, IObserver`1 observer, Action`1 deactivate)
System.Reactive.Linq.QueryLanguage.<>c__DisplayClass421_0`1.<When>b__0(IObserver`1 observer)

in ObservableJoinsTest.cs#L1596

The larger question is, should the following be possible with And in the first place?

var source = Observable.Range(1, 5);

When(
    source.And(source).Then((a, b) => a + b)
);

[akarnokd]

The RxJava version of the join patterns, which was directly ported some time ago, does not have this problem (due to this) and the example above correctly generates 2, 4, 6, 8, 10.