Incorporate statebox/idris-ct into Travis + Nix build

[epost]

Parts of this codebase have been factored out into https://github.com/statebox/idris-ct. This is now a dependency of the current repo (idris-statebox-core), and should be incorporated into the CI builds as such.

• Open source idris-ct to make this possible? Not sure this is necessary but we do plan to do that soon.
• Activate the idris-ct repo on Travis.
• Incorporate idris-ct into this repo's build.