unused import
- Add the a ``sort_type`` keyword argument to ``IIndexSort.sort`` and
  ``IResultSet.sort`` methods.  This value can be passed by calling code to
  control the type of sorting used.

- Add two constants: ``hypatia.interfaces.STABLE`` and
  ``hypatia.interfaces.OPTIMAL``.  These can be used as explicit arguments to
  the ``IIndexSort.sort`` and ``IResultSet.sort`` ``sort_type`` parameter to
  control the stability of sorting.

- The contructor of ``IResultSet`` now accepts a ``sort_type`` keyword
  argument.

- The ResultSet constructed by ``IResultSet.sort`` will be passed the value
  ``hypatia.interfaces.STABLE`` in its constructor to ensure that the second
  and subsequent sorts of the result set will be done as a stable sort, unless
  a explicit ``sort_type`` value is passed to that second sort.

- Fix some tests that were trying to use the since-removed ``_use_nbest``
  attribute of field indexes.

- Remove some tests that were trying to use the since-removed ``_use_lazy``
  attribute of field indexes.