ADT: Avoid repeating iterator adaptor/facade template params, NFC

Take advantage of class name injection to avoid redundantly specifying
template parameters of iterator adaptor/facade base classes.

No functionality change, although the private typedefs changed in a
couple of cases.

  - Added a private typedef HashTableIterator::BaseT, following the
    pattern from r207084 / 3478d4b1, to
    pre-emptively appease MSVC (maybe it's not necessary anymore but
    looks like we do this pretty consistently). Otherwise, I removed
    private
  - Removed private typedefs filter_iterator_impl::BaseT and
    FilterIteratorTest::InputIterator::BaseT since there was only one
    use of each and the definition was no longer interesting.