This PR changes the plugin so it will notice an insoluble constraint of the form Sem r a ~ Foo, and mark that r takes part in a genuine type error. The plugin will then provide a bogus evidence term for IfStuck (IndexOf r _) _ _, which prevents the AmbiguousSend error message from firing.
This PR adds doctests allowing us to write tests for the custom type errors. Having this stuff reified in the test suite means we can iterate on the implementations and give much better QA.
