Flatten dependencies

This PR pulls dependencies in from a .xcworkspace file, allowing something with multiple direct and/or indirect dependencies on Mantle 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