Automatically implement Upcast<Self> for all types

[jnbooth]

Suppose a user creates a function like this:

fn do_something<T: Upcast<MyObject>>(object: &T);

It should be possible to call do_something(object) where object is &MyObject.

The Upcast<Self> implementations are all no-ops.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (5d3b2de) to head (9bb1823).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1247   +/-   ##
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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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[LeonMatthesKDAB]

Thank you @jnbooth.
Together with the changes that @BenFordTytherington is working on in #1252 this means we'll have to use somewhat ugly/verbose disambiguation syntax to specify the type to upcast to, but that's just how it has to be I guess.