Fix Swift 5.7 build errors

[rgoldberg]

Fix Swift 5.7 build errors.

I used Swift.CustomStringConvertible instead of CustomStringConvertible because of a recommendation in the @retroactive Swift Proposal.

I know that you might want to move off of Swift 5.7 sometime, but can you please delay doing so until after a PR for #679 has been merged & released? Dropping support for 5.7 should require a major version bump.

Resolve #706

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rauhul]

@swift-ci test