I say this in the issue, but just for clarity:

* This WOULD be a breaking change to the library API, allowing me to reorganize things a bit, to be more careful about deps appearing in the public API
* This WOULD NOT be a breaking change of the wire format: you could have a postcard 1.0 device talk to a postcard 2.0 device no problem

Essentially, I am proposing an **API** change, but not an **ABI** (or "wire format") change.