Rpcq

This release contains some minor changes to the "core" message spec.

Bugfixes
---------

- Warning serialization (gh-45) was not done in a backwards-compatible way, due to our earlier decision to switch to using dataclasses for rpcq's messages. This patch release edits our serialization/deserialization code to make message spec changes backwards compatible again going forward.

Minor release: makes error handling in the Python Server object optional, so that exceptions raised by the Server itself or by its spawned Tasks can be caught by the Server's owner.