ENH: stop potential HashPtrTable memory leaks (#1286)

- in previous versions HashPtrTable<T> used a HashTable<T*> for its
  underlying storage with additional routines for deallocating memory
  when items were deleted or the table was cleared.

  This generally works well, but direct use of any HashTable<T*>
  removal methods without additional wrapping results in memory leaks.

  The safer means is to use a container with its own memory management.
  The updated version now uses HashTable<autoPtr<T>> for its internals.
  Memory cleanup now automatically, regardless of the access pattern.