Organize code base

[binyamin555]

In the current project layout, all functionalities written in Python are placed in the package's __init__.py file.
This makes things compact on the one hand, but a bit messy on the other hand.

We should probably split the functionalities, so each one has its own file.

Mainly:

• Builtin-extensions
• Extension tools (extension decorator and the extend_type_with function)
• Core utilities (i.e. the functions which should not be used by users, such as get_builtin_type_dict, which, BTW, should be renamed)