Expose Tuple class

For many of the instances, we use a transformation to and from a nested
tuple shape. For example, we can have an instance of `Eq` for any HKD by
converting it to nested tuples and using _that_ instance.

@pacak notes that, while this is fine in principal, not exposing it
means that we can't write functions on top of various internals that are
suitably polymorphic because we can't add this class as a constraint.

This commit exposes this class, and frees us all.

higgledy.cabal

@@ -1,7 +1,7 @@
 cabal-version:       2.4
 
 name:                higgledy
-version:             0.2.0.1
+version:             0.2.1.0
 synopsis:            Partial types as a type constructor.
 description:         Use the generic representation of an ADT to get a higher-kinded data-style interface automatically.
 homepage:            https://github.com/i-am-tom/higgledy

src/Data/Generic/HKD/Types.hs

@@ -27,6 +27,8 @@ module Data.Generic.HKD.Types
 
   , HKD_
   , GHKD_
+
+  , Tuple (..)
   ) where
 
 import Data.Barbie (ConstraintsB (..), FunctorB (..), ProductB (..), ProductBC (..), TraversableB (..))