Add Transitive casting and testing code

[BenFordTytherington]

Currently WIP

Any impl of MainCast<Base=QObject> seems to cause a conflict with the blanket implementation and the generated implementation of Upcast, which is perplexing given the trait bounds, but it does seem to only be QObject.

Testing with QAbstractListModel shows that lib types can work with this code.

Additionally, with one trait impl, it is only possible to combine 2 levels, e.g. A->B ^ B->C => A->C, and additional impls would be needed to extend this to the desired 5 or 6 levels of casting.

Code in cargo_without_cmake example shows the functionality of the transitive casting.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (a662b2e) to head (3da5070).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           73        73           
  Lines        12634     12634           
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  Hits         12634     12634           

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[BenFordTytherington]

Macro now written in example for testing purposes, I wasn't sure where to put it / export it publically. This will implement casting to all the types written after the first one, provided there is casting between each type, e.g chain_cast(A, B, C, D, E) will, provided A -> B, B -> C, C -> D and D -> can cast, generate A -> C, A -> D, A -> E.

In a future commit, could maybe add more recursion so that this also generates B -> D, B -> E, etc... Also probably needs some testing.