PERF: Override equals in IndexVariable

[max-sixty]

• closes PeriodIndex causes severe slow down #1255
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

I tried adding some tests but couldn't find a good way of doing that

There is a 20x speed-up though (see issues for prior):

In [51]: indexes = [pd.PeriodIndex(start=str((1776 + i)), freq='A', periods=300) for i in range(50)]
In [53]: das = [xr.DataArray(range(300), coords=[index]) for index in indexes]
 In [54]: %timeit xr.concat(das)



10 loops, best of 3: 70.2 ms per loop

[max-sixty]

I don't think the AppVeyor break is related to this

[shoyer]

This already has pretty good test coverage in test_variable.py, so I think this is fine.

[shoyer]

I'd prefer equiv is not None, which is slightly more idiomatic for checking default values (I'd rather not rely on the truthiness of arbitrary functions/callables)