Pure Caching

[lewisjkl]

Leaving this mostly as a placeholder for now with details to follow, but basically want a pure alternative to the current mutable-map-based CachedSchemaCompiler.

This will offer several advantages:

• Caching in the presence of transformations (currently cache keys are references, so transformations do not result in stable references)
• Injecting arbitrary/custom behavior into existing SchemaVisitors in user-land will be more possible due to recursion being abstracted to a different layer. Currently this sort of customization requires object-oriented "extending" of SchemaVisitor implementations.