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 (64d4ee6) to head (28db8bd).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1247   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           73        73           
  Lines        12612     12612           
=========================================
  Hits         12612     12612           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.