API: add reads_json, reads_msgpack, reads_csv, reads_pickle as convience functions which accept a string
related #5655
related #5874
closes #5924

todo:

```
[ ] update io.rst docs / whatsnew
[ ] more tests (esp pickle/csv) for passing string to ``read_csv`` or file-handle to ``reads_csv``
```

Not super thrilled with so many new functions... Any chance we could add a
string= kwarg to existing instead?
 closing in favor of #5954 or doing nothing (better IMHO)
