- Modified the core AvroType subclasses to produce schemata such that a nested record schema always becomes a union including that record schema and a reference, where a reference is itself a record of one field called id of type long.
- Modified the signatures for the internal overloaded definition of AvroTypeIO.write and AvroTypeIO.read to facilitate reference resolution. These methods now take a mutable.Map[Any, Long] and a mutable.Seq[Any], respectively.  Implementation and updates in-progress.
- Reorganized the test directory/package structure to be more consistent.