+We return the matching family instances and the type instance at which it
+matches. For example, if we lookup 'T [Int]' and have a family instance
+
+ data instance T [a] = ..
+
+desugared to
+
+ data :R42T a = ..
+ coe :Co:R42T a :: T [a] ~ :R42T a
+
+we return the matching instance '(FamInst{.., fi_tycon = :R42T}, Int)'.
+