Restructure and update repository to work with modern Swift Package Manager conventions - **means no more Carthage support**

[hamchapman]

This updates the repo to use modern Swift PM conventions, which allows us to remove a load of code that was about making tests visible on Linux. It also allows us to remove the xcodeproj file.

However, I wanted to call this out in a separate PR because it means no more Carthage support, AFAIK, because Carthage relies on there existing a public scheme (and therefore an xcodeproj file) in the repo.

Does anyone using the library still use it via Carthage?

[hamchapman]

@martinreichart as someone who has recently contributed, are you consuming the library using Carthage? Just trying to gauge the impact this might have

[martinreichart]

@hamchapman thank you for asking - no we're not using the library via Carthage but with SwiftPM