Add NonEmptyDMap --- depends on #26

[Ericson2314]

Need this to curry maps. Example usage forthcoming

CC @cgibbard

[ryantrinkle]

This looks like it would impose a very large maintenance overhead going forward; I'm not convinced it's worth it. One very valuable thing about the way this package is currently constructed is that it is nothing more than a simple search-and-replace of containers - I've even considered building it from the containers source using a script. How can we bring down the maintenance cost to something reasonable?

[Ericson2314]

@ryantrinkle make containers itself do the same thing! :D. In serious, I can make the containers issue now that this exists enough to quantify the overhead and call out the benefits with tree rotations. Hopefully the existing non-empty containers packages are good enough to point to as a need, but I'd make the version of vessel using this too to bolster the argument.

[ryantrinkle]

Sounds good! I think the main thing is that the containers issue needs to go first; I don't want to merge this here in the hope that we'll eventually become maintainable.