'expanding' type: allow all iterator classes to be used as online accumulators

[ajcr]

The classes should have a window_type='expanding' mode. I.e. the window grows with each new value that is added from the iterator.

There would be no need to keep track of seen values as nothing is removed from the window.

(Later Note): for some algorithms this may not be possible (e.g. median) or worthwhile (e.g. any, all). I need to think further whether it's worth it.