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



ping when fixed / green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=h1) Report
> Merging [#16514](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ef487d9e474e8052c0f7c6260de5802a950defad?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `85.71%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16514/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16514      +/-   ##
==========================================
- Coverage   90.79%   90.79%   -0.01%     
==========================================
  Files         161      161              
  Lines       51063    51065       +2     
==========================================
  Hits        46365    46365              
- Misses       4698     4700       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.63% <4.76%> (-0.01%)` | :arrow_down: |
| #single | `40.15% <85.71%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.99% <85.71%> (-0.08%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=footer). Last update [ef487d9...1de16b6](https://codecov.io/gh/pandas-dev/pandas/pull/16514?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 1) Got a small linting error in `pytables.py` (hence the Travis failure)

2) Can you provide some timing info in the PR given that this is a performance change? @jreback I think this is ready to go? @Kiv Some timing info in the PR would be useful for future reference.  Plus a benchmark test would be good here. @Kiv lgtm. if you would add an asv would be great (you can use your original example, just not as long, maybe 20 nodes or something would be fine). can you add an asv? My git branch got out of date and I don't know how to update it properly. New version with ASV bench is here:

https://github.com/pandas-dev/pandas/pull/16666 in the future see: http://pandas.pydata.org/pandas-docs/stable/contributing.html#combining-commits. Much much better to push to the *same* PR. opening new ones is rarely needed (for the same issue)

you just needed to do:

```
git rebase -i upstream/master
git push yourremove yourbranch -f
```