DEPR: remove freq attribute of Timestamp?
xref https://github.com/pandas-dev/pandas/issues/15141#issuecomment-273184725

In theory these should be completely fungible. If so, we should remove the ``freq`` attribute
of ``Timestamps`` and instead return ``Periods`` in those instances.

```
In [2]: t = Timestamp('2010-01-03 00:00:00', freq='W-SUN')

In [3]: t
Out[3]: Timestamp('2010-01-03 00:00:00', freq='W-SUN')

In [4]: t.to_period()
Out[4]: Period('2009-12-28/2010-01-03', 'W-SUN')

In [5]: t.to_period().to_timestamp(how='start')
Out[5]: Timestamp('2009-12-28 00:00:00')

In [6]: t.to_period().to_timestamp(how='end')
Out[6]: Timestamp('2010-01-03 00:00:00')
```