Porting code to Use C# 8 Nullable Reference Types

[moh-hassan]

This PR fixes the issue #699.
It is a co-work, started by @atifaziz in PR #582 (when C# 8 was still in beta)
then completed by @Orace and @moh-hassan when c#8 is becoming released resulting this PR.
The PR is rebased with the last changes in master.

The PR fix the Nullable Reference Types of c# 8 and make MoreLinq C#8 nullable Ready.

Notes:
No change in the public API, except annotating the null parameters in the next two methods:

  public static IEnumerable<KeyValuePair<int, TSource>>
           IndexBy<TSource, TKey>(
               this IEnumerable<TSource> source,
               Func<TSource, TKey> keySelector,
               IEqualityComparer<TKey>? comparer)    //nullable

   	public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext, 
   Action<Exception>? onError = null,  //nullable
   Action? onCompleted = null)   //nullable

[atifaziz]

@moh-hassan, @Orace Thanks a lot for you help with this. Since this PR builds on PR #582, it would have been best to make your additions via another PR on the respective branch in my fork. Otherwise, it becomes very difficult & time-consuming to review here (as it includes the sum of all changes on master) and consolidate changes between this and #582.

[atifaziz]

I am closing this in the hope that a new PR will be submitted via my fork.