- relocate the pair_entry (HashTable) and unary_entry (HashSet) into
  the Detail namespace and add output handling.

  The output handling at this level removes the reliance on zero::null
  output (HashSet) and allows direct support of pointers.
  This means that the following now works

      HashTable<T*> tbl;
      os << tbl;

  It also means that we don't need to overload operator<< for
  HashPtrTable anymore.

- avoid delete/new when calling HashSet::set(). If the entry already
  exists there is no reason to remove it and add another one with the
  same content.

STYLE: HashTable iterators now have a val() method

- identical to the object() iterator method, but shorter to type.

- applies to HashTable and Linked-Lists

- deprecate the Linked-List iterator found() method, since this makes
  less sense (linguistically)

- can be used as a more natural test on the iterator.
  For example, with

     HashTable<..> table;
     auto iter = table.find(...);

  Following are now all equivalent:

    1.  if (iter != table.end()) ...
    2.  if (iter.found()) ...
    3.  if (iter) ...

- make single-parameter construct (label) explicit
- consolidate iterators
- slightly reduced overhead for some HashSet types
- improved resizing behaviour
- compact output for empty Ptr hashes