Add Internal module

[CedNaru]

Phase 2 of splitting the current godot library into several modules.

This time, I added a new module that is meant to be used as an "implementation" dependency for the core layer. Many previously internal classes were made public and moved to that new module. It means that the core library will be able to freely use them, but they won't be visible to the users.

Some classes had to be slightly modified to take into account that change because they previously referenced each other but are now in separate modules, which means that the reference can only be in one direction. It's actually a good thing, as it helps improve the quality of our code by removing coupling between classes. On that matter, the important changes are between MemoryManager (now in the new module) and KtObject (still in core).