Add an `Eq` instance on GHC 8.6+

[int-index]

GHC 8.6 offers quantified constraints, meaning we can try to do something like this:

instance (forall a. Typeable a => Eq (f a)) => Eq (TypeRepMap f)

[chshersh]

@int-index Could you recommend good tutorial on QuantifiedConstraints?

[int-index]

https://ryanglscott.github.io/2018/06/22/quantifiedconstraints-and-the-trouble-with-traversable/

Here's a three-part blog post series on QuantifiedConstraints from Ryan Scott.