BUG: {expanding,rolling}_{cov,corr} don't handle arguments with different index sets properly

Closes https://github.com/pydata/pandas/issues/7512.
Actually, unless I completely rewrite the expanding functions not to use the corresponding rolling function, even if expanding_xxx() aligns the objects, rolling_xxx() will still try to re-align them, so I think it may be a bit inefficient. I think the current implementation is probably cleanest in this regard.