Flatten dependencies

This PR pulls dependencies in from a .xcworkspace file, allowing something with multiple direct and/or indirect dependencies on Objective-Git and its dependencies to include them all without being required to include multiple copies in Xcode. This can greatly improve the responsiveness of Xcode when first opening a project, as well as making it much easier to determine which copy of a dependency is actually going to be included in the final result.

- [x] Add a workspace for standalone development
- [x] Pull dependencies from the workspace
- [x] Write clear documentation as to why this was done and how to work with it
- [x] Ensure the proper functioning of CI