Revise module and class structure

[penguian]

The module and class structure should be revised again with respect to best practices for Python 3.6+. See (e.g.) Fluent Python.

[penguian]

Examples:

1. containers.List should inherit from collections.UserList.
2. The saveable.Saveable mixin class should always be directly inherted by user classes, rather than via another superclass.