Early/late binding to data source

[Brian Goetz]

We've already committed to a model where stream operations do not 
tolerate modification of their data source during the operation.  For 
example, this is illegal:

   collection.stream().forEach(e -> collection.removeAll(e));

We've also tightened down the spec on "source sharing" and "stream reuse".

We now have to define what "during" means.  We make a substantial effort 
to not touch the data until the user wants to do something with it.  So, 
what should happen in this case:

   List<String> strings = new ArrayList<>(Arrays.asList("a"));
   Stream<String> s = strings.stream();
   strings.clear();
   s.forEach(System.out::println);