add NSMutableArray.SubList features

Currently you cannot extract a sublist from a mutable array and add it to another collection:

```java
NSMutableArray<E> array = …;
Collection<E> collection = …;

SubList<E> subList = array.subList(startIndex, endIndex);
collection.addAll(subList);
```

This patch adds that feature, corrects the value for count() and implements a useful toString method.