[DO NOT REVIEW OR MERGE] Move pyrsistent serialization and hashing helpers to a new module.

[wallrj]

Here's an idea for re-organizing the serialization stuff so that I can e.g. use wire_encode from _diffing in #2839.

Unfortunately it's not quite enough.

Perhaps instead, wire_decode should accept an acceptable_classes argument rather than having the global SERIALIZABLE_CLASSES variable.
After all we'd never expect to find _diffing classes in the control service configuration which also uses the wire_encode format.

Needs more thought.

[Azulinho]

I think you can do a lot better than this, seriously