Add minimal java.util.Arrays implementation required by scala.collection.

Three parts of the Scala collection library uses Arrays:

 - Arrays.fill is used by FlatHashTable and HashTable:

   scala/collection/mutable/FlatHashTable.scala:    else java.util.Arrays.fill(sizemap, 0)
   scala/collection/mutable/HashTable.scala:    else java.util.Arrays.fill(sizemap, 0)

 - Arrays.sort is used by SeqLike.sorted and is implemented using the
   native JS array sort method. Access to the underlying Array is
   special-cased so that code uses dot-select as suggested by Sébastien.

   scala/collection/SeqLike.scala:    java.util.Arrays.sort(arr.array, ord.asInstanceOf[Ordering[Object]])

With this and support for java.lang.String.endsWith the following
partests run (when the parallel part of hashset is disabled).

 - run/hashset.scala
 - run/comparable-comparator.scala
 - run/colltest1.scala
 - run/mutable-treeset.scala