Namespace boogaloo

This PR burns down a lot of our namespaces, which didn't really help anything. We still keep ReactiveUI.Winforms and ReactiveUI.Xamforms because you can use them in conjunction with the core ReactiveUI.

@rikbosch, this PR had to rename the WinForms `ViewModelViewHost` and `RoutedViewHost` because it conflicted with the XAML one - thoughts on the new names? (`ViewModelControlHost` + `RoutedControlHost`)

### TODO:

- [x] Make sure @rikbosch says :shipit: