Spliterator update

[Brian Goetz]

After some offline discussions with Doug, we agreed that the 
getNaturalSplits method can be eliminated completely (rather than 
replaced with an isSplittable as suggested), and instead renaming 
split() as trySplit() which would be allowed to return null if the data 
source was not splittable.  This eliminates the need to spec (and check) 
interactions between isSplittable and subsequent calls to split.  This 
has been committed to lambda repo.