RFC: Implement java.util.Arrays method needed by scala.collection

This is an attempt to implement especially Arrays.sort which is required by the SeqLike.sorted method.

I've marked this as RFC, since the sorting is done using the native js.Array.sort method. In order to do this I had to copy the code from java.lang.reflect.Array.getUnderlying to access the underlying JS array. :-(

The good news is that with these two additional patches, 3 more partests now pass:

- run/colltest1.scala
- run/comparable-comparator.scala
- run/mutable-treeset.scala