Spliterator.tryAdvance

[Remi Forax]

Playing a little bit with how findFirst/forEachUntil can be implemented 
on top of a Spliterator,
I think that tryAdvance should be changed to be able to return a value 
produced in the middle of the consumer taken by tryAdvance.

I would like to have tryAdvance to be like this:

   /**
    * Sentinel value used by tryAdvance to signal that there is no more 
element.
    */
   public static final Object END = new Object();

    /**
      * If a remaining element exists, performs the given action on it,
      * returning the result of the function, otherwise returns {@code END}.
      *
      * @param action The action to perform.
      * @return {@code END} if no remaining elements existed
      * upon entry to this method, else the return value of the action.
      */
    Object tryAdvance(Function<? super T, ?> action);

and forEach is a little bit uglier:
     Function<? super T, ?> action = element -> { 
consumer.accept(element); return null; }
     do {} while(tryAdvance(action) != END);

with that, there is no need to use a side value to express the fact that 
we have already found the resulting value because we can return it has 
the return value of tryAdvance.

cheers,
Rémi