Fix #974: Implement java.util.UUID

Re-implementation of java.util.UUID in Scala in order to be smoothly integrated with scala-js.
Again, this is unnecessarily inefficient.

1. Use `Character.digit(c, 16)` to parse a character
2. Construct the two `Long`s directly.

Note of coding style: always a space after a comma, and never a space after a `(`.