WIP: Experimental changes in `rolling_var` related to #7900

Added logic to `rolling_var` to detect windows where all non-NaN
values are identical.

Need to assess both correctness and performance impact.
`nobs` was a double before my changes (see [here](https://github.com/pydata/pandas/blob/v0.13.1/pandas/algos.pyx#L1067)). I seem to recall that I tried changing it to `Py_ssize_t` and there was some performance degradation, so I let it be. But I may very well be making this up, don't remember the exact details...

Since the effect of this new modifications on performance have to be measured, I'll try things both ways and add some information on the commit message if it stays a double.