Idiomatic data structure for agreement.

Please see the README on GitHub at https://github.com/jcranch/agreeing#readme

agreeing

A simple data structure helping us ask questions of the following sort: "does all this data have the same /BLANK/ and if so what is it?"

For example:

    doTheseHaveTheSameLength :: [String] -> String
    doTheseHaveTheSameLength l = case foldMap (Somebody . length) of
      Somebody n -> "They all have length " <> show n
      Nobody     -> "The lengths differ"
      Anybody    -> "You didn't give me any strings"

This can of course be done with Maybe (Maybe x) instead, but doing so runs the risk of getting confused: which is Nothing and which is Just Nothing?