RoundTripEnvelope should use a more compact data format

[muse254]

Problem

If you compare the encoded data for file assets that is sent encoded to JSON, we have a scenario where ~300kb file asset becomes a ~2MB json payload.
A good approach would be to create or use custom binary encoding formats to optimise for size. This would be perfectly backwards compatible with the current API we have in place.
More on this: https://stackoverflow.com/questions/49096721/json-transfer-vs-binary-transfer

[muse254]

succeeded by globe-and-citizen/layer8-primitives-rs#5