Add "die" word

- This is another word that seems to be useful in several places.  Use
  it to iterate all DIE's instead of winfo, which has now been
  dropped.  Use it to iterate DIE's of a CU, and use it to access a
  DIE associated with an attribute.  The latter used to be done with
  parent, but that doesn't work, because we got at the attribute by
  saying "attribute", not by saying "child".