Add NonEmptyMap and NonEmptySet to containers

[Henning Thielemann]

On Thu, 18 Apr 2019, John Cotton Ericson wrote:

> In https://github.com/haskell/containers/issues/608 I proposed adding 
> non-empty variants of Map and Set, analogous to Data.List.NonEmpty for 
> List, to containers. semigroupoids demonstrates the many uses and 
> structure of non-empty containers in general, and libraries such as 
> https://github.com/mstksg/nonempty-containers and 
> https://github.com/andrewthad/non-empty-containers demonstrate the 
> interest in non-empty maps and sets in particular.

I have also my implementations:
https://hackage.haskell.org/package/non-empty-0.3.1/docs/Data-NonEmpty-Set.html
https://hackage.haskell.org/package/non-empty-0.3.1/docs/Data-NonEmpty-Map.html

Interesting to know, that the implementation in containers could already 
provide this functionality with little changes.