Optional Show instances

[cdsmith]

Currently, many predicates make an arbitrary choice whether to require a Show instance or not. If a Show instance is required, then the Predicate can give better explanations, but can't be used as often. This isn't really a choice that's about the predicate, but rather about the type the predicate is used with.

By relying on https://hackage.haskell.org/package/if-instance, it would be possible to defer this decision to the call site, obtaining a Show instance that can be used if it's available, but otherwise falling back to something with worse messages but that still works! That's perfect. However, it's imperfect that if-instance depends on GHC 9.0, and requires that clients use a GHC plugin.

I think the right thing to do here is to create a different package that depends on explainable-predicates, reexports the same Predicate type, but defines the predicates to use optional Show instances via the plugin. Then users who can tolerate a GHC plugin and are using GHC 9.0 could depend in the new package instead.

Notes:

• In the long-term, it would be very nice if explainable-predicates made it into GHC. However, that would still mean a delay of years before this package could depend on the functionality because of backward compatibility.
• It would be nice if the implementation here could depend on the more generic implementation using if-instance. That would work, except for old GHC compatibility. Perhaps an upstream contribution could widen the range of GHC versions supported by if-instance. If that included everything as far back as 8.6, I think that would be good enough. But 9.0 definitely isn't.