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.

I have refactored the code quite a bit, see what you think of this, @seth-p. The good news is that the performance differences seems negligible, and may even favor the new implementation, which I really find hard to understand. Following are timings over a random array of `size` double values, using a window of `window` items. Timed values are the minimum of enough repeats to take 0.1 sec. total time:

```
                                       master         new
size:     100, window:    10, time: 4.06e-05 sec. 4.06e-05 sec. 
size:     100, window:    30, time: 4.23e-05 sec. 4.19e-05 sec. 
size:     300, window:    10, time: 4.68e-05 sec. 4.56e-05 sec. 
size:     300, window:    30, time: 4.39e-05 sec. 3.98e-05 sec. 
size:     300, window:   100, time: 5.34e-05 sec. 4.23e-05 sec. 
size:    1000, window:    10, time: 9.48e-05 sec. 8.66e-05 sec. 
size:    1000, window:    30, time: 8.01e-05 sec. 9.36e-05 sec. 
size:    1000, window:   100, time: 7.76e-05 sec. 1.22e-04 sec. 
size:    1000, window:   300, time: 7.72e-05 sec. 1.08e-04 sec. 
size:    3000, window:    10, time: 1.34e-04 sec. 1.63e-04 sec. 
size:    3000, window:    30, time: 1.77e-04 sec. 1.38e-04 sec. 
size:    3000, window:   100, time: 1.67e-04 sec. 1.34e-04 sec. 
size:    3000, window:   300, time: 1.63e-04 sec. 1.59e-04 sec. 
size:    3000, window:  1000, time: 1.29e-04 sec. 1.47e-04 sec. 
size:   10000, window:    10, time: 4.34e-04 sec. 4.12e-04 sec. 
size:   10000, window:    30, time: 4.39e-04 sec. 3.18e-04 sec. 
size:   10000, window:   100, time: 3.55e-04 sec. 2.82e-04 sec. 
size:   10000, window:   300, time: 3.59e-04 sec. 2.89e-04 sec. 
size:   10000, window:  1000, time: 4.66e-04 sec. 3.09e-04 sec. 
size:   10000, window:  3000, time: 4.58e-04 sec. 3.64e-04 sec. 
size:   30000, window:    10, time: 1.25e-03 sec. 1.02e-03 sec. 
size:   30000, window:    30, time: 8.98e-04 sec. 7.80e-04 sec. 
size:   30000, window:   100, time: 8.39e-04 sec. 1.09e-03 sec. 
size:   30000, window:   300, time: 8.08e-04 sec. 8.19e-04 sec. 
size:   30000, window:  1000, time: 1.06e-03 sec. 6.99e-04 sec. 
size:   30000, window:  3000, time: 7.99e-04 sec. 6.68e-04 sec. 
size:   30000, window: 10000, time: 6.61e-04 sec. 6.29e-04 sec. 
size:  100000, window:    10, time: 2.46e-03 sec. 2.25e-03 sec. 
size:  100000, window:    30, time: 2.83e-03 sec. 2.29e-03 sec. 
size:  100000, window:   100, time: 2.63e-03 sec. 2.44e-03 sec. 
size:  100000, window:   300, time: 2.50e-03 sec. 2.54e-03 sec. 
size:  100000, window:  1000, time: 2.41e-03 sec. 2.61e-03 sec. 
size:  100000, window:  3000, time: 2.63e-03 sec. 2.34e-03 sec. 
size:  100000, window: 10000, time: 2.38e-03 sec. 2.24e-03 sec. 
size:  300000, window:    10, time: 8.60e-03 sec. 8.85e-03 sec. 
size:  300000, window:    30, time: 8.44e-03 sec. 7.96e-03 sec. 
size:  300000, window:   100, time: 8.74e-03 sec. 8.11e-03 sec. 
size:  300000, window:   300, time: 8.72e-03 sec. 8.40e-03 sec. 
size:  300000, window:  1000, time: 8.45e-03 sec. 8.09e-03 sec. 
size:  300000, window:  3000, time: 8.76e-03 sec. 8.42e-03 sec. 
size:  300000, window: 10000, time: 9.08e-03 sec. 8.51e-03 sec. 
size: 1000000, window:    10, time: 3.17e-02 sec. 3.06e-02 sec. 
size: 1000000, window:    30, time: 3.21e-02 sec. 2.97e-02 sec. 
size: 1000000, window:   100, time: 3.40e-02 sec. 2.82e-02 sec. 
size: 1000000, window:   300, time: 3.17e-02 sec. 3.05e-02 sec. 
size: 1000000, window:  1000, time: 2.93e-02 sec. 3.12e-02 sec. 
size: 1000000, window:  3000, time: 2.94e-02 sec. 2.75e-02 sec. 
size: 1000000, window: 10000, time: 3.34e-02 sec. 2.81e-02 sec.
```
 can you add a test that validates this new behaviour (well not new, but 'correct')?
 It may take a couple of days until I get around to it, but if we think this is worth adding I will definitely write some tests for it.
 sure no problem

I think definitely worthwhile avoiding pathological cases (because of rounding / precision issues) if possible (as u have shown)
 Agreed, I think it's worth handling degenerate cases accurately.
 how's this coming?
 @jaimefrio status?
 I was having some rebase problems last night, but it is mostly ready, Will get back to it tonight and push a hopefully final version.
 Couldn't solve my rebase issues, so created a new PR #8271. Closing this
