Add Transitive casting and testing code

[BenFordTytherington]

Adds macros for implementing transitive casting, for 3 types, or a longer chain.
For 3 types A, B and C, impl_transitive_cast! will implement casting between A -> C, provided the relations between exist. For a longer chain, use chain_cast!.

These are scoped to cxx_qt due to #[macro_export], but Upcast has now also become unsafe and moved into a casting module, use it like so: use cxx_qt::casting::Upcast.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (5d3b2de) to head (7baae3e).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1252   +/-   ##
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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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