Use generic map merging framework

[jcranch]

It is possible to make the code for merge tactics generic (so it can be used for Map and IntMap without duplication, and could also be used for other map-like data structures in other libraries), and to increase the safety in doing so.

I have created a repository containing both an implementation and a fairly lengthy explanation of how it works.

There are some decisions to be made about how to do it: most noticeably, there is a dependency upon Kinoshita's witherable library. I have demonstrated that this library can be trimmed down heavily, but there are still several possibilities for where it should go: it could go into base, into containers, or remain as a separate library (which would then need to be a dependency of containers).

@tomjaguarpaw has spent some time reviewing these ideas (I'm grateful for feedback on an earlier version); he may be willing to make his thoughts known.

[tomjaguarpaw]

Yes, I found @jcranch's write up excellent and I'd recommend it to the containers maintainers. His idea could prove to be a very nice and practical way of generalising merge functionality.