Support abbreviations

- (abbreviation) yields an abbreviation of the DIE on TOS
- ?haschildren can be used on DIE to determine whether it has children
- label, offset, code, ?AT_*, ?haschildren, attribute are words that
  are applicable to abbreviations.  The latter produces abbreviation
  attribute objects.
- label, offset, form, ?AT_* and ?FORM_* are words that are applicable
  to abbreviation attribute objects.
Drop unused code
More comprehensive error messages on type mismatch

- Mention actual TOS profile alongside the expected one.
- Show selectors in bottom-to-top manner, just like we do everywhere
  else.
- Hide show_expects now that nobody uses it.