Resurrect UpdateManager

This PR brings back the Squirrel.Client namespace, with a few important changes:

* Removes Filesystem Abstractions
* Rx => async/await
* Nuke the AppDomain stuff
* Lose InstallManager, we're going to merge it into UpdateManager

### TODO:

- [x] Get the code building
- [x] Get the tests building
- [ ] Get the tests passing
- [ ] Replicate InstallManager's functionality