Auto-generate InferenceError?

The manually-constructed `enum InferenceError` could be replaced by something like the following in bindgen:
 - [newtype_enum](https://docs.rs/bindgen/0.54.1/bindgen/struct.Builder.html#method.newtype_enum)
 - [rustified_enum](https://docs.rs/bindgen/0.54.1/bindgen/struct.Builder.html#method.rustified_enum)
