Add more robust type difference detection

[liss-h]

Currently metall stores typeid(T).name() and typeid(T).hash_code() alongside the object to check if the type of objects is still the same.

This however does not work when the definition of a type changes, because the hash_code does not consider object layout.

Add a more robust way to detect object changes, detecting all changes is impossible but we can at least try improving it by including the
layout of the type in the hash.