IntervalIndex nbytes computation
Currently, it looks like `IntervalIndex.nbytes` just uses `IndexOpsMixin.nbytes`

```python
@property
def nbytes(self):
    """ return the number of bytes in the underlying data """
    return self._values.nbytes
```

Should it instead be `self.left.nbytes + self.right.nbytes`? This will be faster, and I think more correct.