Collectors update

[Brian Goetz]

>> 4.  Rejigger Partition to return an array again, with an explicit
>> lambda (which will likely be an array ctor ref) to make the array.
>> Eliminated the silly Partition class.
>
> Please don't do that, it's pure evil.
>    public static<T> Collector<T, Collection<T>[]>
> partitioningBy(Predicate<T> predicate, IntFunction<Collection<T>[]>
> arraySupplier) {

I've refactored this to make the partition collectors return 
Map<Boolean, X>.