Make the Avro coder bidirectional for types

[joshsh]

Currently, Hydra's avro coder is bidirectional for terms, but unidirectional for types: Avro schemas are the single source of truth, whereas there is no way to pass Hydra types into the coder as the source of truth. The latter is going to be necessary at some point for the sake of producing non-Avro graph data using Avro. The trick will be to re-use as much as possible of the bidirectional term coder between the Avro- and Hydra-based bicoders.