Remove separation between CLTree and CPTree

[slisson]

The separation just has historical reasons. The CP... classes were serialized using GSON, but after replacing it with a more efficient custom serialization format, the separation is not required anymore.

https://github.com/modelix/modelix.core/tree/main/model-client/src/commonMain/kotlin/org/modelix/model/persistent
https://github.com/modelix/modelix.core/tree/main/model-client/src/commonMain/kotlin/org/modelix/model/lazy