Support negative space in SymbolicObjs

I'd like to make a `SymbolicObj3` which consists of a part, plus mounting holes for it. I can either model the part, and compose it into a bigger design via `union`, or model the mounting holes and compose them in via `difference`. But keeping this as two separate parts is inconvenient; it means the two need to be correctly positioned relative to one another in the calling code, rather than in the defining code. Not a good way to prevent user errors!

AFAIK, there is no blending operation that can union the part and simultaneously carve out the mounting holes. This is extremely desirable to me!