Merge AsyncRx.NET assemblies #1896
                
     Merged
            
            
          
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Resolves #1891
System.Reactive.Async.Concurrency, System.Reactive.Async.Core, System.Reactive.Async.Disposables, System.Reactive.Async.Interfaces, System.Reactive.Async.Linq and System.Reactive.Async.Bcl all merged into System.Reactive.Async
System.Reactive.Shared has been removed because almost everything it did was already in System.Reactive. So we now take a dependency on System.Reactive, and one extension method puts back the one thing that was missing.