API: to_series/to_frame Index methods
xref https://github.com/pandas-dev/pandas/pull/15216#issuecomment-275240736

``Index.to_series()``
``MultiIndex.to_frame()``

should we harmonize? (e.g. make both available to both? for compat), leave it like this?

should also prob add the ``index=True`` kw to ``.to_series()`` for compat.