Fix #974: Implement java.util.UUID

Re-implementation of java.util.UUID in Scala in order to be smoothly integrated with scala-js.
Multi-line method should have braces.

Moreover, prefer this style for chained `if`s:
```scala
override def compareTo(that: UUID): Int = {
  if (this.highBits < that.highBits) -1
  else if (this.highBits > that.highBits) 1
  else if ...
}
```