Showable Unique

[pythonissam]

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[pythonissam]

issue ref: #802

[pythonissam]

@snoyberg Well, can you review my PR?

[snoyberg]

FYI, I don't typically maintain persistent on a regular basis, so I'm not the best person to review PRs.

I was going to ask why a major version bump, but upon reflection I agree that it has the potential to cause compilation breakage even in the absence of orphan instances.