Scala.js support #45

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

• Check dependencies
  • shapeless
  • Cats
  • circe
  • scodec
  • scalacheck-shapeless
  • 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 Add tests for compatibility between Scala versions #44 for testing)