Support multi-file changes

The `WatcherHotReloadService` can generate deltas for an entire solution. So we should be able to support:
1. Changes to multiple source documents from a single project
2. Changes in multiple projects in a solution.

There are a couple of issues:
* [ ] Need to support `-msbuild:foo.sln` not just `-msbuild:foo.csproj`
* [ ] extend the json format for multi-file updates
* [ ] extend the DeltaProject machinery (maybe add a DeltaSolution?) so that we can dispatch deltas of different projects to different places.
* [ ] get rid of the single distinguished "output assembly" (ie the baseline of the .csproj that was passed to us on the command line). Right now all the deltas end up with: (1) their name based off the name of that assembly, (2) in the same directory as that assembly.

* [ ] For the live runner, we could debounce the FileSystemWatcher events and group the file updates together.