Map should implement Set-like features based on key

[jmalloc]

• isIntersecting
• is[Proper][Sub|Super]set
• union (similar to merge, except do not clobber)
• intersect
• difference
• symmetricDifference

[jmalloc]

Removed from milestone, I'm not sure exactly how to approach this at this stage.

Perhaps AssociativeInterface / MutableAssociativeInterface should cherry pick some of the methods from SetInterface. Alternatively, sets could be implemented as/on top of associative collections much like the STL.