Allow client code to tailor Kryo.

[sageserpent-open]

There are several workarounds in the code for specific classes that tend to be referred to in client domain models. This will only get worse in future.

Let's all client code to register serializers (and possibly instantiation strategies) via callbacks or something else that doesn't fully expose with Plutonium does with Kryo, but allows some tailoring of the latter.

We need to be careful that fancy inter-item serialization in 'ItemStateStorage' plays well with this.