Add supported for polymorphic nested Rel8ables (#215)

```haskell
data Nest t u f = Nest
  { foo :: t f
  , bar :: u f
  }
  deriving (Generic, Rel8able)
``

src/Rel8/Generic/Rel8able.hs

@@ -63,6 +63,11 @@ type KRel8able = K.Rel8able
 -- This is almost 'Data.Type.Equality.==', but we add an extra case.
 type (==) :: k -> k -> Bool
 type family a == b where
+  -- This extra case is needed to solve the equation "a == a" (where a is a
+  -- KRel8able), which occurs when we have polymorphic Rel8ables
+  -- (e.g., newtype T t f = T { x :: t a })
+  (a :: KRel8able) == (a :: KRel8able) = 'True
+
   -- This extra case is needed to solve the equation "a == Identity a", 
   -- which occurs when we have polymorphic Rel8ables 
   -- (e.g., newtype T a f = T { x :: Column f a })

tests/Rel8/Generic/Rel8able/Test.hs

@@ -5,6 +5,7 @@
 {-# language DuplicateRecordFields #-}
 {-# language FlexibleInstances #-}
 {-# language MultiParamTypeClasses #-}
+{-# language StandaloneKindSignatures #-}
 {-# language TypeFamilies #-}
 {-# language UndecidableInstances #-}
 
@@ -171,9 +172,17 @@ data NestedTableTestB f = NestedTableTestB
   deriving anyclass Rel8able
 
 
-
 newtype IdRecord a f = IdRecord { recordId :: Column f a }
   deriving stock Generic
 
 
 instance DBType a => Rel8able (IdRecord a)
+
+
+type Nest :: KRel8able -> KRel8able -> KRel8able
+data Nest t u f = Nest
+  { foo :: t f
+  , bar :: u f
+  }
+  deriving stock Generic
+  deriving anyclass Rel8able