Use case: diff two blueprints

[rpdelaney]

Pretty much the title. Given two blueprints, produce a "diff" that indicates which entities are present in one and not in the other, and vice versa.

[rpdelaney]

I just remembered the main reason I wanted this. It would be killer for designing upgradeable blueprints. For instance, a blueprint that consumes 45 iron ore/sec and makes steel can be extended to weave in a second belt of ore. With something like this, I could do a "before" and "after" blueprint, subtract one from the other, and there's my overlay.