This is an attempt to resolve the circular recursion case, in this PR:

1. Rework the state to store the descriptor once, so symbols reference a filename instead of holding a duplicate descriptor
2. Some state simplification, making some operations stricter
3. Add a function that shrinks the state by combining cycles

When files participate in a cycle, causing the reflection tree to instead be a graph, we combine those participating file descriptors into a single descriptor.

• minimal change from each-module-as-a-file
• clients should now resolve schemas that contain cycles