This avoids `naive_solver` tearing whole goals apart, even if the goal
appears exactly as a hypothesis.

This closes issue #36.

Now we follow Coq's stdlib and declare this instance using a `Hint Extern`;
this avoids making `flip` type class opaque.

Revert "`RelDecision` instance for `flip`, and make `flip` tc opaque to avoid loops due to eager unification."

This reverts commit b81aa3aa.

`RelDecision` instance for `flip`, and make `flip` tc opaque to avoid loops due to eager unification.

Otherwise type class search ocasionally unfolds them and finds
wrong instances. Based on an issue reported by @jihgfee.