Use Splat to provide features previously done in ReactiveUI

This PR makes ReactiveUI officially depend on [Splat](https://github.com/paulcbetts/splat) for service location, logging, and unit test detection.

## Find And Replace Breaking Changes

* `RxApp.DependencyResolver` => `Locator.Current`
* `RxApp.MutableResolver` => `Locator.CurrentMutable`
* `RxApp.InUnitTestRunner` => `ModeDetector.InUnitTestRunner`

## More Breaking Changes

* `IEnableLogger` now needs a `using Splat;`
* `InitializeResolver` => `InitializeSplat` + `InitializeReactiveUI`