ENH: Add set_closed method to IntervalIndex
xref https://github.com/pandas-dev/pandas/pull/21584#discussion_r198317876

This method would allow for changing the `closed` value of an existing `IntervalIndex`.

Example usage:
```python
In [2]: index = pd.interval_range(0, 3, closed='both')

In [3]: index
Out[3]:
IntervalIndex([[0, 1], [1, 2], [2, 3]]
              closed='both',
              dtype='interval[int64]')

In [4]: index.set_closed('neither')
Out[4]:
IntervalIndex([(0, 1), (1, 2), (2, 3)]
              closed='neither',
              dtype='interval[int64]')
```