ENH: optionally eliminate duplicates on hashedWordList construction (issue #375)

- 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.