This class is largely a pre-C++11 holdover. It is now possible to
simply use move construct/assignment directly.
In a few rare cases (eg, polyMesh::resetPrimitives) it has been
replaced by an autoPtr.
- makes it easier to use as a wordHashSet replacement for situations
where we want to avoid duplicates but retain the input order.
- support construction from HashTable, which means it works like the
HashTable::sortedToc but with its own hashing for these keys.
- expose rehash() method for the user. There is normally no need for
using it directly, but also no reason to lock it away as private.
