Await die die fing die

This PR kills Microsoft.Bcl.Async (and by proxy, ReactiveUI 4.x) with :fire:. Always code in Anger.

### TODO

- [X] Murder anything that starts with Microsoft.Bcl
- [X] Replace all awaits with Rx
- [X] Move a bunch of stuff from RxUI 4.x into ReactiveUIMicro
- [ ] Rewrite Setup UI to not use RxUI.Routing or other stuff I don't like