[IMP-dev] Moving container-related code to its own module

I'm proposing moving the container-related code out of core into their own module. This is something I'd thought about when I first implemented them, but had decided not to, without any strong reason. The reasons for wanting to do this now are: - they form a well defined collection of functionality that can be best explained together - the list of functionality in core is very long and hard to one's head around - core is huge, takes forever to compile

Affected classes would be List*Container, *ContainerSet, AllPairContainer and kin and ClosePairContainer and kin. I think few people use the classes directly currently, other than to create ListSingletonContainers, so there would not be too many changes.

Thoughts? Objections?