- Eliminated the `io` subproject (moved all classes into `core`)
- Removed the `io` method on the AvroTypeIO type, added a lazy `io` value to the AvroType type.
- Moved conversions from AvroType to AvroTypeIO from AvroTypeIO.Implicits to AvroTypeIO, got rid of the inner Implicits object.
- Refactored use sites to reflect the above changes.