Modules for new map merge functions may be renamed

We hope to make one more, fairly small, major release for containers for inclusion in GHC 8.2. I am considering renaming the new modules Data.Map.Merge.Lazy and Data.Map.Merge.Strict instead of Data.Map.Strict.Merge and Data.Map.Lazy.Merge. That seems more consistent with the module scheme for the rest of the package, and leaves open the option of adding extra merge-related modules in the future. I'm sorry about the potential wobble, but I think taking care of this now will lead to much less trouble than waiting till later.