Implement improved dup() and copy()

[joente]

Description

When creating a duplicate or copy with a large deep value, potentially many things will be created when a thing contains a self reference. This pull request improves duplicates or copies by using a single copy for the same thing if found by recursion. This can drastically improve the performance and is usually what you want as a user.

Type of change

• Improvement of existing functionality. While we do not expect existing code depends on the old behavior, the new behavior is different so you might want to check where copy() or dup() is used with a deep level greater or equal to two. For example my_thing.dup(2) might yield a different result.

How Has This Been Tested?

• Update test code

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes