Added arranged methods for Threshold, Distinct and Count

[Kixiron]

Adds methods that return (arbitrary) arrangements for the threshold, distinct and count family of functions

[Kixiron]

Any updates on this PR?

[frankmcsherry]

I'm sorry for the delay. I don't see this merging. It seems to primarily contain a number of "one-liners" that seem easy to handle in user code, without needing special support from the DD library itself. If that is wrong for any of these, please do call them out, but I believe most of these are wrappers around flavors of reduce_*, and if they are useful to your project 100% wrap them up as extension traits. But at the moment, I'd rather have a smaller public surface area to better manage expectations about what is stable.