Rework combine

[kMutagene]

We need to find a way to perform a deep copy before combining. That way, most mutation-related issues are solved.

Continuation of #35 , which originally aimed to canonize Combine and Copy. However, these functions are slightly different from each other, and Combine needs some more work. Notably, it was possible to provide a combine function that keeps a derived type inheriting from DynamicObj. This will be lost when performing a deep copy, as that returns a boxed DynamicObj.

I think we need DeepCombine and ShallowCombine respectively, maybe some API that also helps with the subsequent unboxing, e.g. DeepCombineAs<'T>