standalone Serializer/materializer: how to extend TypeCodes as a client?

[MarcusDenker]

Soil adds a typecode for PersistentDitionary, this class is not part of the Serializer package.

Problems:

• Serializer depends on that class
• When loaded first it is nil and the TypeCode tables are intialized at that point with nil instead of the class
• We have to call "SoilTypeCodes initialize" in SoilPersistentDictionaryTest>>#testCommitAndRead to make it work

Could we provide a way for Soil to register it's own TypeCodes instead?