wit-bindgen: Only emit error type impls in one place

[elliottt]

Feature

When a type is used in an error position in a result in multiple interfaces, it gets an impl std::error::Error definition in each module that it's used in. It should only get a single impl in the module that defines it instead.

Benefit

Error type reuse would be easier between wit interfaces.

[alexcrichton]

Ah ok I don't think that duplicate impls are generated, just a confusing rustc error message -- this should be fixed by #7075