Add the :class:`curses.complexstr` type, an immutable string of styled
character cells (the counterpart of :class:`curses.complexchar`), and the
:mod:`curses` window method :meth:`~curses.window.in_wchstr` that returns one.
The string-cell methods :meth:`~curses.window.addstr`,
:meth:`~curses.window.addnstr`, :meth:`~curses.window.insstr` and
:meth:`~curses.window.insnstr` now also accept a :class:`~curses.complexstr`.
Like :class:`curses.complexchar`, it works whether or not Python was built
against a wide-character-aware curses library.
