Fix #974: Implement java.util.UUID

Re-implementation of java.util.UUID in Scala in order to be smoothly integrated with scala-js.
If you don't implement the method, remove it entirely rather than using `???`. Like you wrote, you get errors at runtime. By removing the method, you get errors at link time, which is much better.

(I suggest commenting the signature, so we remember that there's something missing here.)