Consider changing to reference assembly-style API

[jwosty]

It might be nice to have a (contract) reference assembly for Interstellar (which mostly just shadows Interstellar.Core, which is pretty much just a set of interfaces anyway). The various Interstellar hosts don't really share much implementation anyway. This would be similar in spirit to .Net Standard, and Xamarin.Forms.

It would have to be made clear what the advantage of doing this would be over just having everything program against Interstellar.Core interfaces, as we currently do. Is there any advantage this would give? Not clear.