Add DMap similar to the one from dependent-map

[3noch]

https://hackage.haskell.org/package/dependent-map-0.2.4.0/docs/Data-Dependent-Map.html provides a "heterogeneous" map type, allowing keys to determine the type of the values.

[nikita-volkov]

I think it would be better for someone to implement it as a separate package, I would rather not take it on for maintenance. If a lower-level interface will be needed, the internals of "stm-containers" are now exposed by the "stm-hamt" package.