PERF: HDFStore __unicode__ method
supersedes #16514 

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

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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=h1) Report
> Merging [#16666](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5a6f50d039e0b01dd835ee5401282ab58b22c534?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `94.73%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16666/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16666      +/-   ##
==========================================
- Coverage   90.93%   90.93%   -0.01%     
==========================================
  Files         161      161              
  Lines       49269    49268       -1     
==========================================
- Hits        44802    44801       -1     
  Misses       4467     4467
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <5.26%> (ø)` | :arrow_up: |
| #single | `40.22% <94.73%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `93.04% <94.73%> (-0.08%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.36% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16666?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/16666?src=pr&el=footer). Last update [5a6f50d...5d2812d](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=h1) Report
> Merging [#16666](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5a6f50d039e0b01dd835ee5401282ab58b22c534?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/16666/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16666      +/-   ##
==========================================
- Coverage   90.93%   90.92%   -0.01%     
==========================================
  Files         161      161              
  Lines       49269    49271       +2     
==========================================
  Hits        44802    44802              
- Misses       4467     4469       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <4.76%> (-0.01%)` | :arrow_down: |
| #single | `40.22% <85.71%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `93.04% <85.71%> (-0.08%)` | :arrow_down: |
| [pandas/io/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsLnB5) | `80.55% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16666?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/16666?src=pr&el=footer). Last update [5a6f50d...5d2812d](https://codecov.io/gh/pandas-dev/pandas/pull/16666?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
