S2RegionIterator

I have a need to iterate over every cell within a region using `GetInteriorCovering` and `GetCovering`. I don't need the minimal cell set.

Using the existing `GetInteriorCovering` and `GetCovering` functions doesn't work because they calculate and store all of the cells. In my use case, this requires gigabytes of memory and leads to a crash. As an alternative, I've altered these functions to take a functor/lambda as an argument which performs the needed processing. The result is fast and has limited memory requirements.

I wonder if it is worth trying to upstream this?