More Data.Set and Data.Map operations

[Henning Thielemann]

On Mon, 8 Aug 2016, David Feuer wrote:

> So what names would you prefer?

Good question. I would prefer something with takeWhile and dropWhile, 
however that also only matches if the predicates are monotonic. We already 
have mapMonotonic. Maybe:

    takeWhileMonotonic
    dropWhileMonotonic
    spanMonotonic