Remove apron dependency and cache work somewhere.

[ftc]

Please correct me if I am wrong but my understanding is that the Apron work is put on indefinite hold for now. In light of this is there a way we can we set things up so we can check out/compile/test develop without installing Apron?

We should figure out a way to retain this code without having it add to the complexity of using our library. I propose we move it into either a submodule or another project which relies on cuanto as a dependency.

[bechang]

Working decision: split cuanto into sub sbt projects