Clean up IonSchemaError and IonSchemaResult constructor functions

[popematt]

ion-schema-rust should implement a similar approach as what is described here:

🗺️ IonFailure (imported above on line 5) is a new crate-visible trait that provides extension methods for the IonResult and IonError types.

Previously, each IonError variant had one helper function in crate::result that would build an IonResult, and an accompanying function that would being an IonError. The names got pretty weird. For example, decoding_error would return an IonResult (not an error, as one could reasonably infer), and decoding_error_raw would return an IonError. (The "raw" indicated that it hadn't gone through the final step of wrapping the IonError in a Result::Err, which was not an obvious connection.)

The IonFailure trait replaces those method pairs. Now you can use IonResult::decoding_error to make an IonResult and you can use IonError::decoding_error to make an IonError. Much more straightforward.

Originally posted by @zslayton in amazon-ion/ion-rust#584 (comment)