Make `as-edn` faster

[cddr]

The function as-edn could be faster I think. Currently, we take an avro GenericData, write it out to JSON, then parse the JSON into an edn object. Nice and simple but incurs an unnecessary serialization and deserialization when all we really need is a "recoding" of an object already in memory.

https://avro.apache.org/docs/1.4.1/api/java/org/apache/avro/io/Decoder.html

We might be able to do better by implementing an EdnDecoder or something. Have a look at abracad for potential inspiration.