The orphan rule is defined here:
https://docs.carbon-lang.dev/docs/design/generics/details.html#orphan-rule
**Orphan rule:** Some name from the type structure of an `impl`
declaration must be defined in the same library as the `impl`, that is
some name must be *local*.
Update tests that were running afoul of the orphan rule unintentionally.
Add tests that do violate the rule intentionally and test edge cases.
