Graph schemas

[robrix]

I'd like to be able to provide some sort of schema for the kinds of nodes I'm constructing within a graph, and where they can be found in relation to the syntax tree.

(This would give us a nice way to describe stack graph or eval rules for example, in a way that's machine-checkable.)

Talked with @dcreager about this, and he suggested testing against such as a midway point to actually verifying.

In addition to verification/validation, this could also be used to give us constructors for subgraphs, which could be quite convenient when building stack graphs for example.