BUG: nanops.var produces incorrect results due to int64 overflow (fixes #2888)
fixes #2888

as a side note, apparently the tests as they were would never fail for certain ops, because they were comparing (e.g.) `Series.std()` to `np.std()`, but the latter was just calling the former anyway. (maybe this is new behavior since the tests were created.)

anyway, the tests are doing the right thing now and all pass
 ok to merge on this?
 yep
