Cleanup [SERINA-10]
Add getMapperClassFromModel() [SERINA-10]

Simply standardising the way Mapper classes are derived from their
corresponding models
Add another usage of getMapperClassFromModel() [SERINA-10]
Add registry, a place to store spawned Mappers [SERINA-10]

This is a simple flyweight registry of spawned mappers within the Query
class, to stop having to spawn new ones if old ones already existed.