Multibinder should accept bindings to sets of elements

_From [mattmccutchen@google.com](https://code.google.com/u/100074016002343462025/) on October 21, 2011 17:12:58_

Multibinder should accept bindings to sets whose size is not known until injector creation and then add all the elements of the sets to the multibound set.  Contrived example:

Multibinder.newSetBinder(binder(), User.class, MyAnnotation.class)
  .addSetBinding().toProvider(HappyEngineersProvider.class);

class HappyEngineersProvider implements Provider<Set<User>> {
  `@`Inject Company company;

  `@`Override public Set<User> get() {
    Set<User> happyEngineers = new HashSet<User>();
    for (User e : company.getEngineers())
      if (e.isHappy())
        happyEngineers.add(e);
    return happyEngineers;
  }
}

_Original issue: http://code.google.com/p/google-guice/issues/detail?id=662_