more efficient methods than fallbacks, e.g. any and all

[goretkin]

For my own code, I've defined:

# TODO arrr type piracy
Base.any(p::PaddedViews.PaddedView) = p.fillvalue || any(parent(p))
Base.all(p::PaddedViews.PaddedView) = p.fillvalue && all(parent(p))

I was about to make a PR for consideration, when I realized there are quite a few reduction methods that could benefit from this: sum, prod, minimum, maximum, extrema. Instead of writing a specialization of all of them, it would be possible to write a specialization of mapreduce with some helper functions that provide basic facts that repeated + is * and repeated * is ^ and repeated min/max is Identity.

Then I also figured there could be definitions that specialize on scalar--PaddedView broadcasting operations, which would preserve the padding structure. This would be a breaking change, since the return type of e.g. padded_array .== 2 would change from OffsetArray to PaddedView.

It seemed a bit never-ending to encode all the rules, but I imagine at least some of these are worthwhile, so I opened this issue for discussion.