Add some notion of correctness to problems

[artasparks]

We should add some methods to indicate correctness for problems.

Historically, the way I've done this is to add a GB[1] property to the nodes. https://github.com/Kashomon/glift-core/blob/master/src/rules/problems.js has more ideas -- it's even more flexible. A problem is correct if some number of conditions are satisfied.