CompositeAsyncDisposable not available

Hi,

I recently came across a situation where `CompositeAsyncDisposable` wrapper might come in handy, however it turned out the AsyncRx package is still not published (https://github.com/dotnet/reactive/issues/1118). I ended up copy-pasting the code I needed, but of course that's not the ideal solution.

Is there any chance you could integrate it into the regular Rx NuGet package before the AsyncRx package is released? That'd be helpful.

Cheers