Feat: Support incremental operations with Declarative Shadow Variable

[triceo]

For groups, we typically have to do things like Collections.max(). This requires us first to build a collection, and then to iterate over the collection to provide a result. Also, this happens for every variable individually, as opposed to once per group. The new design should:

• Introduce an API similar to constraint collectors, which can perform arbitrary operations incrementally.
• Introduce implementations of that API for common use cases, such as min/max.
• Introduce some sort of a caching mechanism, so that each member of the group can get the precomputed result, as opposed to each computing it all over again.