Fix issue #1627

[pratapsingh1729]

Fixes #1627 (Verus crashing on an external impl Trait when that trait has an associated type).

@Chris-Hawblitzel or whoever wrote the trait logic should definitely confirm this, but below is why I think this fix is sound.

In rust_to_vir_impl.rs:608, we look up trait_map[&trait_path]. Here, trait_map contains all the traits in the current crate, while trait_path is iterating through auto_import_impls, which contains all impls that are either for "new" traits (traits from the current crate), or external impls from the current crate. Then, the for loop starting on line 576 iterates through all of the auto_import_impls (the impls that might be visible to Verus), looking up their traits in the trait_map.

The error case in #1627 comes from an external impl for an external trait, so the external trait doesn't appear in trait_map. But since it is an external impl for an external trait, Verus shouldn't be able to see it, so I think it shouldn't need to check whether there is an associated type match for it in the list of local traits.

If my reasoning is incorrect, please let me know---I'd be happy to implement a different fix!

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.