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`.
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
ENH: TimedeltaIndex and Timedelta scalar support (GH3009)

BUG: coercion of timedeltas in assignment (GH8209)

TST: skip sql.timedelta tests for now

COMPAT: provide conversion for passing in non-ns datetime64/timedelta64

BUG: diff on Dataframe with timedelta64 broken (GH4533)
fix from comments