PERF: HDFStore __unicode__ method
HDFStore __unicode__ now only returns file path info. New info() method has the previous behavior of __unicode__.

```python
import pandas as pd
store = pd.HDFStore('test.h5', 'w')
for i in range(5000):
    store.put('table_{}'.format(i), pd.DataFrame([i]))

# Before
%time str(store)
CPU times: user 26.1 s, sys: 156 ms, total: 26.2 s
Wall time: 26.2 s

# After
%time str(store)
CPU times: user 0 ns, sys: 0 ns, total: 0 ns
Wall time: 40.1 µs
```

 - [x] closes #16503
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry


