ENH: partial reorganization of HashTable internals (#1160)

- 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.