refactor: readers/writers stored in TypeDescriptor

[markehammons]

This is less safe than storing the Readers/Writers in the ReadWriteModule due to the possibility of mixing runtimes/modules, but we don't have a well-engineered solution to that at present anyway.

On the plus side, this move reduces lookup time of the readers/writers, the cost of storing them, and allows me to explicitly control how they're accessed/updated wrt concurrency.