Switch to quantified constraints, and eliminate entailment logic

[conal]

Now that GHC has quantified constraints, the tedious constraint entailment logic can probably be eliminated from the implementation. See a similar conversation.

[mikesperber]

I should note I have a patch(-in-progress) for this for the core classes. Needs work in the plugin, however.

[mikesperber]

I've pushed the WIP on this here:

https://github.com/mikesperber/concat/tree/quantified-constraints

[mikesperber]

Note that this is awkward and requires CPP, as explained here:

https://mail.haskell.org/pipermail/haskell-cafe/2021-September/134395.html

[mikesperber]

ghc ticket here:

https://gitlab.haskell.org/ghc/ghc/-/issues/20318