Scala.js support

[durban]

We should support Scala.js where it makes sense (e.g., the model checker probably cannot, but it isn't need to anyway).

• [x] Check dependencies
  • [x] shapeless
  • [x] Cats
  • [x] circe
  • [x] scodec
  • [x] scalacheck-shapeless
  • [x] ScalaTest
• [ ] Figure out what sbt configuration is needed
  • [ ] make sure existing workflow is not broken (e.g., Eclipse)
  • [ ] releasing should work too
  • [ ] sbt-plugin should work too (even for Scala.js client projects)
• [ ] Fix code where needed (e.g., see #44 for testing)

[durban]

Consider using https://github.com/sjrd/sbt-dynscalajs.