Refactor ExtensionsKHR.cpp to remove duplicated code from Serialize.cpp and Deserialize.cpp

[mebrunn]

The Parse* and Serializer* functions in ExtensionsKHR.cpp are copied from Serialize.cpp and Deserialize.cpp. That code should be shared rather than duplicated.

[vorlov-playrix]

It becomes even worse when you try to add your own extension (de)serializer. You have to copy these functions to the new module in order to use them.