Reduce the number of AsyncRx.NET NuGet packages

[idg10]

AsyncRx.NET currently has several projects corresponding to areas of functionality which, in Rx.NET, were all merged into the single System.Reactive package back in Rx v4.0.

Although we think that 'great unification' may have gone too far, the problems we see there are all to do with the decision to include UI-framework-specific code in that unification. We don't see any problems with the decision to merge concurrency, disposables, interfaces, LINQ, and subjects into a single library.

Given that AsyncRx.NET doesn't currently have any UI-framework-specific features, there seems to be no good reason not to merge it all down to a single package, to be consistent both with where Rx is now, and where we are aiming for Rx to be in V7.0 once we've moved the UI-framework-specific pieces back out again.