Caliburn.Micro 4.0.0-alpha.1

This release is earlier that I'd normally I'd like as it's not yet feature complete, however a lot of people are asking for versions that support .NET Standard. Given this I've pushed out this release early, and after a bit of thought I suspect it will be a good thing in the long run.

The major "theme" for 4.0.0 is "async", moving most major interfaces such as IActivate, IDeactivate, IGuardClose etc to async implementations. Naturally this will involve a lot of breaking changes as I move forward. Potentially rolling out early alpha releases at various steps can lessen the blow for people wanting to be on the bleeding edge. The first interface up for changes is IEventAggregator and is included in this release.

I've started documenting the changes and migration path to 4.0.0 which I encourage you all to read.

If you're interested in what's in the pipeline for 4.0.0 you can view the project on GitHub as well as the associated milestone.

If you do upgrade and run into any issues please log them quickly (with a reproduction if possible) and I'll see what I can do.