BUG/ENH/API: ewm* debias, 'adjust', min_periods; roll/exp_var/std 1 val
Allow deprecate_kwarg to transform arguments

Add a keyword argument `mapping` to `pandas.util.deprecate_kwarg` to allow
remapping deprecated keyword arguments.  For example, in #7963, which inspired
this PR, one could write this:

```python
@deprecate_kwarg(old_arg_name='infer_dst', new_arg_name='ambiguous',
                 mapping={True: 'infer', False: 'raise'})
def tz_localize(self, tz, ambiguous=None):
    ...
```

Tests of this functionality are added to `pandas/tests/test_util.py`, created
for this purpose; the `if __name__ == '__main__'` section is cribbed directly
from `test_index.py`.
BENCH: add benchmarks for SQL
Merge pull request #8102 from jorisvandenbossche/sql-bench

BENCH: add benchmarks for SQL
CLN: fix whitespace issue in vb_suite/io_sql.py
API: Series.to_csv() should return string when path is None

Test cases for when path is None

Return string when path is None

Add note to release notes under API

Add comments to test cases
Merge pull request #8220 from onesandzeroes/csv-output

API: Series.to_csv(path=None) should return string to match DataFrame.to_csv()
numexpr is now on github
Merge pull request #8221 from zachcp/patch-1

DOC: numexpr is now on github
DOC: fix formatting error in io.rst

Lines had an extra space causing the code block above to keep going for the rest of the note.
Merge pull request #8223 from isms/patch-1

DOC: fix formatting error in io.rst
Merge pull request #7991 from ischwabacher/patch-2

Allow deprecate_kwarg to transform arguments
Merge pull request #7926 from seth-p/ewmvar_bias_correction

API/BUG/ENH: ewmvar/cov debiasing factors; add 'adjust' to ewmvar/std/vol/cov/corr; ewm*() min_periods