Avro is self describing [1]. It embeds the schema of whatever it is describing inside the format so a reader can handle arbitrary avro serialized files. It is also cross platform. I'm not sure if avro maps can cache keys, but if you use a record type the keys are not stored in the data. As far as I know though, avro cannot handle custom types, so those will need customized importers and exporters.

[1]: http://avro.apache.org/docs/current/