Fixes #117 - The project does not compile with Xcode 12

[st3fan]

This patch brings the project up to date with Xcode 12. It now cleanly compiles except for two innocent warnings about simulator versions (for testing) in the AliSoftware/OHHTTPStubs framework.

Because AliSoftware/OHHTTPStubs had to be upgraded to a newer version, I used that as an opportunity to kick Carthage out of this project and instead us the Swift Package Manager. This makes the project simpler and more standard.

I also let Xcode upgrade the recommended project settings, but this included an upgrade of the DEPLOYMENT_TARGET to 12.0 - Focus (the only consumer of this library) still deploys to 11.4 so I changed that back in the project.

We can probably not land this until Focus is upgraded to Xcode 12.

Cc @boek