More candy from Hush

* We're avoiding mutability a lot in Hush, which would explain some of
  the seemingly trivial categories on dictionaries.

* The usefulness of each of these in a generic sense is open to debate
  if the reviewer considers these to be too specific to Hush.
> This will make sure that the feature is available on NSSet as well.

This can be done even right now (by checking for `respondsToSelector:@selector(enumerateObjectsUsingBlock:)`).

The problem with these duck typing checks is that we'll have to add extra code to ensure that we don't end up comparing an array with a set etc. Not a big deal implementing them, but I think we should YAGNI here.