Fix #974: Implement java.util.UUID

Re-implementation of java.util.UUID in Scala in order to be smoothly integrated with scala-js.
Well, good point, type inference is good, except for:
a) the mask for `node`: it needs to be told that it is a long (compiler error),
b) the mask for `timeLow`: if not explicit, it leaves the highest bits to 1, e.g. the following result (from tests)
```
Expected 'ffffffff9479f2a3-479c-b621-ccf1-f89a67784932' to be '9479f2a3-479c-b621-ccf1-f89a67784932'.
```